Speaking of Color - Part 2

"A Study in Scarlet"

We can identify a color by its attributes: hue, hue purity, and luminance. For instance, we can describe one part of the image above as “a light, very intense red,” another as “a rather dark, dull red," and so on. Some modern color models adopt this way of sorting colors.

Let’s explore the three attributes and consider how some of our ordinary words for them take on technical meanings in modern systems for organizing color.

In a previous post, “Speaking of Color—Part 1,” we distinguished perceived color and psychophysical color. Roughly, that’s how something (either a light or a reflective surface) appears to us and how a system specifies a color as a member in its color space.

The color words we’re talking about in this post can do double duty. They are used by everyone to identify and compare perceived colors, but some are employed (with different, technical meanings) in some color models to define a range of psychophysical colors.

By becoming more aware of how these terms are used in ordinary usage and modern color models, we can understand and more effectively use the color tools in digital painting software. We can make better paintings.

Sorting Colors by Hue

In ordinary language we often use “hue” interchangeably with “color,” saying things like “The mountains take on a lovely hue in the alpenglow” or “Why does she dye her hair that hue?” In modern color models, however, “hue” has a technical meaning that departs from common usage in two ways. First, it names just one of the three attributes of color. When these color models specify a color by three dimensions—the sort of color it is, how colorful it is, and how dark or light it is—“hue” names just the first attribute, the one that allows a color to be sorted as red, or blue, or mauve, and so on.

There are a bazillion hues, or sorts of color. The color systems usually arrange them as a spectrum around the circumference of a circle, the ends of the spectrum being joined at the non-spectral hue of magenta. There are many advantages to arranging hues in a circle. For instance, it visually depicts the fact that hues are qualitatively more or less different from one another: that is, some of them, like red and orange, take our perception in a similar direction, but others, like red and green, take it in quite different directions.

We don’t have common names for many hues. To remedy this, the color systems that stay close to ordinary naming practices designate a few key hues with common names (e.g., red, yellow, green, cyan, blue, and magenta) and identify other hues either as mixtures of them (e.g., in the Munsell Color System intermediary hues are called “red-yellow,” “yellow-green,” and so on) or as resembling them (e.g., in the Natural Color System “R90B” names a red-blue with 10% resemblance to red and 90% resemblance to blue).

However, the color systems normally used in digital painting software, the HSL and the HSV (or, sometimes, HSB) models, simply number the hues. They usually parse the hue circle into 360 sections and number them from 0 to 359.

Rebelle HSL (left) and HSV (right) Color Pickers set to Hue 0

Because choosing from that many hues can be distracting to a painter, Rebelle software allows users to simplify the spectrum to just 36, 24, or 12 numbered hues. In Rebelle, hue 0, which in the HSL and HSV models is the red hue used in color monitors, remains in the 10 o’clock position on the hue circle, no matter how the spectrum is divvied up. Yellow remains at the top.

(Occasionally you will see HSL and HSV/HSB color pickers that divide the hue circle into units labeled from 0 to 100%. This derives from how the original HSL and HSV/HSB models identified hues, as a percentage of the way around the hue circle, starting from 0 for monitor red.)

The square regions in the center of the color pickers above reflect a second way the technical meaning of “hue” departs from ordinary usage. The many colors within those squares (except the white, black, and gray colors) have the same hue; they are all red tones! “A Study in Scarlet” above is made with tones of hue 0 in the HSL system. They differ from one another in some degree of hue purity and luminance, the second and third dimensions of color experience.

Sorting Colors by Hue Purity and Luminance

Painting software can indicate the color in play by displaying its dimensions on three sliders: for example in the HSL model, one slider shows the hue angle or number, another shows saturation percentage, and a third shows lightness percentage; or in the sRGB model, one slider shows the amount of its red component (0 to 255), another shows its green, and a third shows its blue. You could use these sliders to slightly modify the color in play, switch to an analogous color, or choose a contrasting color, and so on. But it would not be easy to anticipate what the result will be!

That’s why color pickers supplement that basic information with graphical representations of the dimensions. Because a computer display is two-dimensional, color pickers usually show two of the color dimensions in one graph and the third one in a separate graph: some common solutions are a square (for color purity and luminance) within a hue circle, a triangle (for color purity and luminance) within a hue circle, a square (for color purity and luminance) next to a line of hues, a square for the red and green components next to a line for the blue component, and so on.

In the color pickers above, the horizontal axes in the squares represent a measure of hue purity and the vertical axes signify a measure of luminance. Once again, since there are distractingly many choices in those dimensions (the default is 65,536 options), Rebelle software allows users to impose a grid with just 25, 49, or 81 options.

The measures of hue purity and luminance differ in the HSL and HSV/HSB color models. Both models use “saturation” to measure hue purity but disagree on its meaning. And from what I’ve read in careful studies (e.g., David Briggs’ The Dimensions of Colour web book, especially sections 1.1, “Colours in Space,” and 7.4, “Hue Circles Based on Additive Complementary Pairs,” and Alexei Boronine’s critique of the HSL color model), the HSL and HSV/HSB terms for hue purity and luminance depart from common usage and from technical meanings in the perceptually based systems (like Munsell Color System and Natural Color System) that are preferred by designers.

There are other problems of a practical kind. Two colors with the same HSL lightness number may appear quite different in lightness. In either system, two colors with the same saturation number may not look equally colorful. And the distance between two hues on the hue circle is not a reliable predictor of how similar or different they appear. For such reasons, these studies warn against using HSL or HSV/HSB pickers uncritically to investigate color relationships or design harmonious color palettes.

Nevertheless, the HSL or HSV/HSB color pickers are useful for making small adjustments in color as you paint. The HSL picker offers a wider range of tints (lighter tones of the hue) and the HSV/HSB picker provides more shades (darker tones). When I use these pickers, I prefer a moderate number of options on them: 36 hues on the circle and 49 choices in the square:

Rebelle HSL Color Picker with Grids

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Thanks for reading!

I hope that you enjoyed this post and that it inspires you to enjoy digital painting. If you find this post helpful, please share it with your friends. And please send me your insights on digital painting and suggestions for Digital Paint Spot.

Bob Kruschwitz