Making a useful LCh color palette
This tutorial provides a set of downloadable LCh-based color palettes to use with GIMP-2.10. The color palettes were put together using information from the handprint website. The LCh color wheel provides a useful framework for organizing and choosing colors, and allows to access the treasure-trove of colorimetric color information available on the internet. Links are provided to some nice articles for skin colors and flower colors.
The last section of this tutorial discusses how it's possible that there could be such a thing as an unprintable color in the very small sRGB color space, and compares results of using HSV Saturation vs LCH Chroma to lower the saturation of colors that are out of gamut with respect to a printer profile: lowering the Chroma produces much nicer results.
Written May 2018.
How to make useless color palettes
Two useless color palettes:
An easy way to make an sRGB color palette is to:
- 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.
- Pick a hue interval such as every 10, 15, or 30 degrees.
- 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:
- 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.
- 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.
- 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:
- 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.
- 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.
- 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: