Making a useful LCh color palette

How to make useless color palettes

Two useless color palettes:

An easy way to make an sRGB color palette is to:

1. Pick an available color space. GIMP mostly only provides HSV and LCh, but some painting and editing programs provide additional color spaces such as HSL and HSY.
2. Pick a hue interval such as every 10, 15, or 30 degrees.
3. Then pick the most saturated possible sRGB color at the specified hue intervals, starting usually at hue=0 and proceeding all the way around the color wheel.

This is an easy way to make a color palette. But it doesn't produce a useful color palette unless your goal is to splash as many highly saturated colors as possible onto your screen:

What is HSV? What is LCh?

HSV is derived from RGB and so varies from one RGB color space to the next. The "every 30 degrees HSV" color palette shown above includes the sRGB color space's Primary, Secondary, and Tertiary cusps. The Primary cusps are at HSV 0, 120, and 240 degrees (sRGB Red, Green, and Blue), and the Secondary cusps are at 60, 180, and 300 degrees (sRGB Yellow, Cyan, and Magenta).

LCh is a polar transform of the LAB color space, which in turn is a perceptually uniform transform of the XYZ reference color space. LCh uses distance from origin (Chroma) and hue angle to locate colors, instead of the perhaps more familiar LAB a-b axis coordinates.

The "every 30 degrees LCh" color palette shown above doesn't include any of the sRGB cusps, though some of the colors land more or less close to some of the sRGB cusps. For example the LCh hue angle 60 is one degree away from the HSV hue angle 30, and the LCh hue angle 300 is one degree away from the HSV hue angle 240. The "every 30 degrees" LCh hue angles in between LCh h=60 and LCh h=300 are increasingly distant from the sRGB color space cusps.

Why both of these "every 30 degrees pick the most saturated colors" color palettes are useless

I bet you thought that I was going to tell you why the HSV color palette is bad and the LCh color palette is good. But actually any color palette consisting of a selection of the most saturated colors in the sRGB color space makes a terrible color palette for painting, for three reasons:

1. Picking a color at hue angles located every "X" degrees around a color wheel might seem like a logical way to build a color palette. But using this approach to make a hue-based color palette doesn't guarantee that the visually most important hues are included in the palette.
2. sRGB colors are additive (transmissive, radiant), which means sRGB colors reach their maximum Chroma at Lightness values that are not representative of the Lightness values at which surface (reflective, subtractive) colors, and specifically paint colors, reach their maximum Chroma.
3. Colors that result from picking the most saturated sRGB colors every "X" degrees around a color wheel vary greatly in saturation, with colors near the sRGB primaries being considerably more saturated than other colors. The resulting inconsistency in saturation skews our perception of the colors by drawing our eyes away from the less saturated colors. I'm not referring to HSV "Saturation", which is a very poor measure of saturation.

Both of the above color palettes are useless. But the HSV color palette is "more useless": In the "pick a color every 30 degrees around the wheel" LCh color palette, the resulting hues are perceptually uniformly spaced, as well as spaced by equal degree increments. But the "every 30 degrees" HSV color palette oversamples yellow greens and violet blues, and undersamples true greens, blue greens, green blues, and true blues.

An example of an actually useful LCh color palette

A useful LCh color palette

The LCh color palette in Figure 2 above solves the problems inherent in using the "every X degrees pick the most saturated sRGB color around a given color wheel" approach to making a color palette:

1. Visually important hues arranged as complementary pairs of colors: The hues in this LCh color palette are based on Bruce MacEvoy's complete palette for watercolor painting. MacEvoy's CIELAB color wheel shows the CIELAB locations of paint pigments mentioned in the "complete palette". Also see the CIELAB hue circle section in MacEvoy's comparison of hue circles.

   MacEvoy notes that we distinguish more colors in the warm hue range from 0 to 90 than we do for the other quadrants on the color wheel. But I added enough hues from the other quadrants to make sure every hue in the palette is paired with its complementary hue.

   Referring to MacEvoy's CIELAB color wheel to locate the average hue angles of suggested paint pigments for given colors in a "complete palette" does introduce "slop/leeway" into the process of picking specific LCh hues for a digital LCh version of the complete palette, but no more than when putting together a complete palette for wet media.

   Also different people draw the lines in different places between one color and the next, for many reasons ranging from personal to physiological to cultural. To delve into the physiological and cultural reasons, do an internet search for unique hues, and for color names. The color palette in Figure 2 no doubt reflects my own preferences for where to draw the lines (and how many lines to draw) between one color and the next. The palette does include all four unique colors at the hues indicated on MacEvoy's CIELAB color wheel, and also includes hues for the positive and negative a* and b* axes.

2. Lightnesses appropriate to surface colors, and specifically to paints and paint pigments. All the hues in the color palette in Figure 2 above have Lightness values appropriate to paints and paint pigments. Lightness values vs maximum Chroma values for sRGB colors vs surface colors below (Section E1) has more information.

3. Consistent saturation values: As far as possible given the gamut limitations of the sRGB color space, all the colors in the color palette in Figure 2 above have their LCh Chroma equal to their LCh Lightness, such that the saturation of the colors is constant from one hue to the next. Section E2 below demonstrates the "lumpiness" of a color palette composed of the most saturated sRGB colors.

Useful, but could be better

The color palette in Figure 2 above has two drawbacks: the colors are too colorful and there aren't any gray patches:

Less colorful colors:

The colors in the color palette in Figure 2 above are not nearly as saturated as colors produced by picking the most saturated possible colors around the color wheel. Nonetheless, these colors are very colorful! A serious problem with staring at "too colorful" colors in a digital color palette is that less saturated colors start to look really insipid, potentially leading to a vicious cycle of adding more and more saturation while painting (and also while editing photographs).

So I made a series of color palettes, keeping the hues and Lightnesses constant, and varying the Chroma, to produce Saturation levels of 1.0, 0.8, 0.6, 0.4, and 0.2. The slideshow below shows the resulting colors, arranged as wedges in a circle:

Gray patches:

Five different LCh color palettes is a bit inconvenient to work with. And personally I find it useful to have easy access to gray colors at pre-selected Lightness values: L=10, L=20, L=30 and so on through L=90, plus L=5, L=15, L=85, and L=95. So for use in my own digital darkroom I combined two of the palettes — the Saturation=0.8 color palette and the Saturation=0.4 color palette — into one color palette, and added the gray patches.

According to MacEvoy's information on paint pigments, "black" paint pigments typically have a Lightness value around L=15-20. But the available "darkest dark" depends of course on the media. For oil pastels (at least the inexpensive brands) "black" has a Lightness value of around L=30 — as MacEvoy notes somewhere on his wonderful website, this tonality doesn't look "black" unless it's surrounded by lighter, brighter colors. Real media "white" (even for cheap oil pastels) is in the range L=90 to 95.

Useful for what?

Personally I never found HSV color wheels and color palettes very useful, neither for picking colors nor for organizing color information, nor for thinking about color relationships — something about the HSV color wheel with its overemphasis of saturated yellow green and blue violet colors just looks wrong to me. But I find myself using LCh color wheels and palettes all the time:

I use LCh color palettes for picking colors for painting and also for split-toning and colorizing black and white renditions of photographs. For example, I was painting a still life of an orange, with the orange sitting next to my computer screen. Matching colors on screens to colors in actual objects is challenging! but GIMP's LCH color space capabilities allowed me to make an LCh "hue series" of bright orange color swatches, and colors with a hue angle near 61 were a good match for the orange.

If you are into the sort of mixed media art that involves scanning "wet media" work and then continues working digitally, or else starts with a print of a digital image and then applies "wet media" paints to the print, then LCh color values can help you pick digital hues that match "wet media" images and "wet media" pigments that match digital hues.

I also use the LCh color space as a framework for organizing thoughts and information about colors. Maybe other people have intuitive understandings of color relationships and a reliable memory for actual colors. But my own understanding of colors is being built up bit by bit from looking, reading, experimenting, painting pictures and recoloring black and white photographs, and back to looking, reading and experimenting.

LCh allows to make use of a vast wealth of color information on the internet, such as Bruce MacEvoy's wonderful handprint website (which answers the question "Where is brown?") and studies that have been done to answer questions like "what color is skin", "what color is the sky", what colors are various fruits and flowers", and etc:

Where is brown? plus colors of skin, colors of sunsets and light sources, and colors of blue skies and shadows

Where is brown?

No doubt you already know this, but still it's something that I find absolutely fascinating: Where is brown on the color wheel? Obviously none of the colors in any of the color palettes shown above are brown. Equally obviously, all of these color palettes span the entire color wheel, so it looks like there's no place to put brown. But actually brown is what you get if you lower the LCh Chroma and Lightness of orange hues.

In Slideshow 2 below, the first slide shows violet-red, red, orange-red, orange, yellow-orange, yellow, yellow-green, and green colors from my LCh color palette, spanning from hue=0 to hue=162. None of these colors look even vaguely "brown". The second slide is exactly the same as the first slide, except that in rows 2 through 5 I lowered the Lightness and the Chroma more or less proportionately, to maintain saturation while making darker colors. The third slide is exactly the same as the second slide, except I lowered just the Chroma (again in rows 2 through 5), to produce more desaturated colors.

This phenomenon of colors completely changing their character as they get darker and less saturated is confined to a fairly narrow range of warm hues. Orange-red through orange-yellow hues (hues 45 through 80, extending perhaps to hues 38 through 90) change to brown. Yellow and green-yellow hues (90 and 100) turn to gold-green. Hues 38 and 90 start to look brown (at least to me) if the color is sufficiently dark and desaturated. But the remaining colors on the color wheel just get darker and less saturated, without also somehow morphing into entirely new colors.

Bruce MacEvoy describes the whys and hows of turning yellows to gold-greens and cheerful warm colors to browns, and also why this phenomenon is confined to such a narrow range of warm hues. See unsaturated color zones for how to produce and manipulate these unexpected colors.

Skin colors

The same LCh hues that make browns, extended a bit to include hues from maybe 20 through maybe 85, also make the colors of skin — try Chromas between 15 and 25, and Lightnesses in the range 30 to 70.

There is a lot of research on skin color available on the internet. Try a search with these terms: "skin color CIECAM OR CIELAB" — values given using CIECAM02 will need to be converted to CIELAB/CIELCh (use ArgyllCMS xicclu). Here are links to some nice articles:

• Skin Color Measurements in Terms of CIELAB Color Space Values
• Investigation of Chinese skin colour and appearance for skin colour reproduction
• Characterising the variations in ethnic skin colours: a new calibrated data base for human skin
• Measuring Human Skin Colour
• An investigation into the variability of skin colour measurements

Colors of sunsets and light sources, and colors of blue skies and shadows

Expanding the range of "hues that make brown" to include reds and yellows allows to make sunset colors. This same expanded "sunset" range of hues also allows to make the colors you'd use to paint warm highlights to suggest the colors of light coming from various light sources.

The complementary hues for the expanded "sunset" range of hues allow to pick colors for painting cool shadow colors that might be cast by objects illuminated by your chosen "color of the light". And not coincidentally these same hues also constitute "blue sky" colors (sRGB Blue is considerably more violet than actual blue skies).

My article Using LCH to pick complementary colors and for making hue-based color harmonies has GIMP-2.10-oriented information on using the LCh color space for picking warm and cool hues for light sources and shadows, and for choosing color harmonies, with links to relevant pages on the handprint website.

Other colors

Whatever subject out there in the real world you might be interested in painting, very likely someone out there has done a study on the CIELAB/LCh and/or CIECAM color values. For example, a quick internet search turned up these articles with information on flower colors and the color of grapes for making red wine:

• Biochemical and colorimetric study of flower color in phlox species
• Natural Variation in Petal Color in Lycoris longituba Revealed by Anthocyanin Components
• Relationship between the Composition of Flavonoids and Flower Colors Variation in Tropical Water Lily (Nymphaea) Cultivars
• Rose Petal Color Determination
• Color and Anthocyanin evaluation of Red Wine Grapes by CIE L*, a*, b* Parameters

I only glanced through the "flowers and grapes" articles. There are some nice scatter plots of Lightness, Chroma and hue values, but you have to scroll past stuff you might not find very interesting. I was surprised (not sure why!) to find that so many really pretty flowers have colors in the violet-blue to red-violet quadrant of the CIELCh color wheel, including some of my favorite flowers. For some reason I never thought of that particular quadrant of the color wheel as having nice or interesting colors, probably because I only ever thought of these colors in their darker and more saturated varieties. As I mentioned earlier, at least for me the LCh color wheel makes a nice framework for storing away color information, and now I have an entry for blue-violet, violet, and red-violet flower colors.

Of course colors in paintings don't have to match real world colors. But I can't help but think that creative departures from reality might work better if one has an idea of what the "reality starting points" might be. And of course having the actual subject to look at is better than looking colors up on the internet or in books. But having reference values helps even when the subject is sitting right in front of you. For example, how much towards yellow does human skin go, and at what Chroma values? How far towards yellow or magenta do red roses go? And so on — reasonable starting colors helps!

Downloadable color palettes plus some other useful stuff

The LCh color palettes zip file has:

1. All five color palettes in the "Saturation series" of LCh color palettes.
2. An LCh color palette with both the Saturation=0.8 and Saturation=0.4 color swatches, plus a set of gray swatches — this is the color palette I use most often.
3. The "useless" color palette with side-by-side HSV and LCH "every 30 degrees most saturated colors".
4. An "every ten degrees around the LCh color wheel" color palette, with Saturation = 1.0 for all hues, within the limits of the sRGB color gamut. This palette has slightly different Lightness values for the hues, with the lowest Lightness at 270 and the highest Lightness at 90. It's a pretty set of colors, though perhaps there are too many swatches for comfortable use when picking colors.
5. Two color palettes for really cheap oil pastels — a set of 24 water-soluble oil pastels, and a set of 25 non-water-soluble oil pastels. Go to your local art or office supply store and you'll know exactly which really cheap oil pastels I'm talking about.

   Even though these pastels have low price tags, they are a lot of fun to draw/paint/color with. I made the color palettes by making heavily-scribbled color patches with each pastel on nice drawing paper, taking a photograph (shooting raw, color-balancing, and setting the white point), and color-picking the resulting colors. The lighting was full spectrum but not even, and I used the paper itself to set the white balance. So the LCh values should be considered no more than approximate.

6. A blank LCh color wheel for designing your own LCh color palettes. Also feel free to download the "color wheel" images from Slideshow 1 above.

Notes

Lightness values at which various hues reach their maximum possible Chroma

When putting together an LCh color palette, picking hues isn't enough. It's also necessary to pick a Lightness value for each hue (and also a Chroma value — see Section E2 below).

Additive vs subtractive colors:

Out there in the real world we encounter a mix of additive (transmissive, radiant) and subtractive (surface, reflective) colors:

• Real world additive colors include rainbows, blue skies, glowing coals, laser beams, backlit leaves and flower petals, and backlit stained glass windows.
• Subtractive colors are seen on surfaces such as leaves and flowers when lit entirely from the front, and on completely opaque surfaces, and of course on painted surfaces and paint pigments.
• Many times the same object will present a mix of additive and subtractive colors, as many objects present a mix of opaque and partially translucent/transparent surfaces that might or might not be at least partially backlit depending on the lighting.

In the digital darkroom, all the colors we see on our screens are additive colors produced by our monitors. When working in an RGB matrix color space (such as sRGB or Rec.2020), the specific Lightness values at which various hues reach maximum Chroma depends on — in fact is entirely determined by — the color space's RGB XYZ primaries. In other words, the specific Lightness values at which various hues reach maximum Chroma in the sRGB color space is not a physical property of additive colors per se, but only a reflection of the primaries that define the sRGB color space (and similarly for all other RGB matrix working spaces).

For the sRGB color space, the Lightness values for the maximum Chroma for a given hue range from a low of L=30 for the sRGB Blue primary (for which the maximum Chroma is 131 at LCh hue=301), to a high of L=98 for sRGB Yellow (for which the maximum Chroma is 95 at LCh hue=100).

Picking Lightness values for the hues in an LCh color palette:

For any given hue, painted (subtractive) colors and sRGB (additive) colors reach their maximum Chroma at different Lightness values. Figure 3 below shows the Lightness values at maximum Chroma for selected hues, for sRGB colors compared to the colors in the LCh color palette shown in Figure 2.

For the warmer hues on the color wheel, sRGB colors and painted colors have reasonably close Lightness values at maximum Chroma, within the color gamut limitations of the sRGB color space. For the remaining part of the color wheel, painted colors reach maximum Chroma at lower Lightness values than sRGB colors:

Printed and painted colors of course are subtractive colors, not additive colors. For the LCh color palette in Figure 2 above I chose Lightness values representative of painted colors at maximum Chroma, using information from these sources:

1. The Value of Peak Saturation, on Bruce MacEvoy's very awesome handprint website. This page provides information for the tonal values at which Munsell (painted) colors reach maximum chroma.
2. Bruce MacEvoy's CIELAB color wheel shows the LCh hue angle and Chroma for various pigments. The same pigments are on his tonal color wheel tonal color wheel, and so a bit of cross-correlation allows to match various pigments at given hue angles with the Lightness values of these same pigments.
3. My own explorations of Lightness vs maximum Chroma for hues around the LCh color wheel, for selected (mostly fine art) printer-paper profiles downloaded from the internet.

The information I used to pick Lightness values for the colors in the LCh palette in Figure 2 above was only sufficient to allow picking approximately appropriate Lightness values for maximum Chroma for painted and printed colors for the hues in the LCh color palette in Figure 2 above. When picking these Lightness values, my twofold goal was to pick reasonably representative Lightness values for each hue, while also producing smooth color transitions from one hue to the next. In other words, for any given LCh hue, somewhat higher or lower Lightness values would also be "in the neighborhood" of the Lightness value of the maximum Chroma for painted colors at the specified hue.

A measure of saturation

This section is not yet finished as I need to make some figures showing maximum Chroma and maximum Saturation for different hues in the sRGB color gamut.

OK, I've thrown the word "saturation" around quite a bit. So what am I talking about?

• From a paper by Fairchild on Color Appearance: Due to the lack of a related chromaticity diagram, saturation is not officially defined in CIELAB. However recalling the [color appearance] definitions of chroma (colorfulness/ brightness of white), lightness (brightness/brightness of white), and saturation (colorfulness/brightness) . . . [then a correlate of saturation for CIECLAB is] Saturation = C*/L*
• Two diagrams from the Munsell Book of Colors showing lines of constant saturation for a violet-blue (purple-blue) color and a blue-green color.

Saturation-wise, a color palette composed of the brightest and most saturated sRGB colors is a very lumpy color palette, with some colors (near sRGB Green and Blue) having extremely high saturations levels, and other colors (near sRGB Cyan, including green-blues and blue-greens) having much lower saturation levels. This is characteristic of RGB color spaces in general. The advantage of working in larger color spaces is that the lower-chroma colors that result from the particular shape of matrix RGB working spaces, still have room to reach higher chromas (compared to smaller RGB color spaces) before hitting the outside edges of larger RGB color spaces.

I haven't yet put together some plots. But to give the most extreme contrast in LCh Chroma values for the most saturated sRGB colors, the Chroma of the sRGB Blue Primary is (which is really a blue-violet color) is 131 with a Lightness value of 30 and an LCh saturation value of 131/30, or 4.4. Whereas sRGB Cyan has a Chroma of 53 at a Lightness value of 91 and a resulting saturation of 53/91, or 0.58. So sRGB Blue is 8.3 times more saturated than sRGB Cyan — that's a rather huge disparity in saturation values! For comparison, the LCh saturation of the sRGB Red Primary is 1.97, and the saturation of the sRGB Green Primary is 1.29. But all the intermediate "most saturated sRGB colors" have lower and sometimes considerably lower LCh saturation values.

An immediate result with practical applications that follows from the shape of RGB matrix color spaces is as follows: No printer (at least not using today's technologies combined with existing pigments) can print all the colors in large RGB color spaces, or even in the very small sRGB color space. But a large RGB color space (which will have a whole lot of unprintable colors) is required to hold all printable colors.

Modifying color palettes to eliminate colors that can't be printed

The brightest and most saturated sRGB colors aren't printable:

Almost none of the most saturated sRGB colors are printable by a typical "big box store" printer such as a Costco Fuji printer. None of these colors are printable by the type of commercial printers used to print magazines, children's books, posters, and such. And most of these colors aren't even printable by a top-of-the-line fine art printer using "best of the best" photographic paper:

So even if your main artistic ambition is painting backlit stained glass windows (very saturated "additive" colors!), if you want to make paper prints of your finished images and you use these most saturated sRGB colors, the resulting prints won't be as colorful as the images on your screen. Relying on Perceptual Intent to "fix the difference" is one way to address this problem. Another option — that gives you as the artist more control over the final printed colors — is make and use color palettes that are at least somewhat tuned to your intended output device's color gamut.

Given how small the sRGB color space is, why aren't all sRGB colors printable? One reason (discussed above) has to do with the odd shape of matrix RGB color spaces in general, with resulting colors near the color space Red, Green, and Blue primaries that are hugely saturated compared to even the largest color gamuts of current fine-art printers and papers.

Another reason even wide-gamut printers can't print all sRGB colors has to do with Lightness values at maximum Chroma for RGB color spaces vs pigments used to make surface colors (also discussed above). A third reason is because printers can't print colors with LAB L values near zero or near 100 (well, with OBA-laden paper, it's possible to print LAB L values that are greater than 1.0). Even the darkest pigments reflect "some light" and even the lightest paper (apart from OBA-laden paper) doesn't reflect 100% of the light falling on it.

Lowering HSV Saturation vs lowering LCh Chroma to make printable colors:

"Printable" of course depends on the specific printer and paper used to make the print. Not too long ago I did one of my periodic internet searches to check the state of current printer technology. From personal preference I excluded looking at the color gamuts of prints made on non-archival paper, high gloss paper, and paper with optical brighteners, and instead looked at fine art printers combined with archival quality fine art photographic paper.

As far as I can tell by soft-proofing to various fine art printer-paper profiles, when combining the Epson P9000 printer with Museo Silver Rag paper, the resulting color gamut is about as good as it gets. If a given sRGB color is outside the color gamut of this particular printer-paper combination, it's not likely to be printable on any currently available fine art printer. So this will be the "sample" printer/paper combination used for the rest of this Section.

Note: My choice of the Epson P9000 printer over similar fine art Canon printers — which I suspect have equivalent color gamuts when paired with the same paper — was mostly by chance, after reading various reviews of printers to ascertain the state of the "latest and greatest fine art printers". As I have no practical experience with these printers, nothing in this article should be construed as any sort of recommendation. But I will say it was easier picking through the Epson website's list of printers than trying to pick through the Canon website to dig out the model numbers for their fine art printers.

OK, what's the best way to modify these highly saturated "not printable" sRGB colors taken at 30-degree intervals around the HSV and LCh color wheels, to make printable colors? An obvious solution is to lower the HSV Saturation for the HSV colors, and lower the LCh Chroma for the LCh colors, until the colors are in-gamut with respect to one's chosen printer plus paper. Let's see which approach works better. The screenshot below shows the result with respect to our "sample" fine art printer/paper:

Looking at Figure 5 above, it's obvious that lowering the HSV Saturation enough to bring the out-of-gamut HSV colors back into the printable gamut of our "sample" fine art printer/paper substantially alters the colors. By contrast, lowering the LCh Chroma enough to bring the out of gamut colors back into the printable gamut produces a lot less visual difference between the out-of-gamut and in-gamut colors.

Compared to the HSV colors, the out of gamut LCh colors weren't quite as far out of gamut to begin with, as none of the LCh colors at the "0/30/60/90/etc" hue angles on the LCh color wheel are located precisely on the primary cusps of the sRGB color space. On an HSV color wheel, the sRGB primaries are at 0, 120, and 240 degrees. The corresponding LCh hue angles for the sRGB primaries are 41, 134, and 301 degrees, respectively. So in the LCH color palette shown above, only the LCh hue at 300 degrees is "super close" to an actual sRGB primary.

Even if you prefer to use HSV color palettes, you might try using the LCh Chroma slider when you need a less colorful version of a given color in your palette. The HSV hue will likely change, but the actual perceived hue won't change. Well, at least for most colors the perceived hue won't change noticeably. But CIELAB isn't perfect, which is a major reason why we have CIECAM02 and why making better color appearance models is such an active area of research.