Speaking of Color - Part 1

We use the word “color” in various ways. For example, after closely examining the painting above we might say, “The two circles appear to be different colors, but they are the same color.” Let’s sort how “color” works in this situation.

To most observers, the circle on the right looks a bit darker and bluer than the one on the left. Indeed, the right circle may look closer in color to the left square than to the left circle. Yet I painted the two circles with the same color in my software. What’s going on here?

Modern color theorists distinguish perceived color from psychophysical color. They would say the two circles are the same psychophysical color but not the same perceived color. The background colors of the two squares makes all the difference.

After we examine this distinction between perceived color and psychophysical color, we’ll consider how a modern color model specifies some psychophysical colors within a color space. Why should we care? Becoming more fluent in the lingo of modern color theory can help us understand and more effectively use the color tools in digital painting software. Then we can make better paintings.

Two Meanings of “Color”

In ordinary situations, we use the word “color” in two ways: (1) to distinguish how things (either lights or reflective surfaces) appear to us and (2) to identify a member in a defined collection of colors. A simple experiment with contrast phenomena will help us sort these two usages.

Paint two squares with two very different saturated colors, and then in the center of each square paint a circle with a third saturated color. (For the best result, the third color should be quite like the color you used for one square, but very different from the other one.) Your experiment with contrast phenomena might look like the painting above.

Now it’s natural to say, “The surrounding field changes the color of the circle,” or “The two circles appear to be different colors, but actually they are the same color.” The first use of “color” (in italics) refers to how a region feels to us: the circle on the left just looks different from the one on the right. The second use of “color” (in bold) refers to a specified member in some color model. For instance, the color in each circle above can be called (86, 128, 255) in a standard RGB color model.

The technical term for the first use of “color” (in italics above) is “perceived color.” The International Commission on Illumination, or Commission Internationale de l’Eclairage (CIE), defines “perceived color” as the “characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colorfulness (or saturation or chroma).” It notes that the perceived color we see “depends on the spectral distribution of the color stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer’s visual system, and on the observer’s experience of the prevailing and similar situations of observation.” (CIE 17-198)

In other words, how a thing (a light source or reflective surface) feels to us varies quite a bit, depending not only on the wavelengths of energy that it emits or reflects, but also on the wavelengths of energy emitted or reflected by surrounding things (as your contrast phenomena experiment shows), the present condition our eyes and optic nerves and brain, and what we were expecting to see given what we’ve noticed before in normal or similar situations.

There’s a technical term for the second use of “color” (in bold above): it’s “psychophysical color,” which the CIE defines as the “specification of a color stimulus in terms of operationally defined values” (CIE 17-197). For example, in the experiment above, the two circles are the same psychophysical color because they have the same specs in the standard RGB color model.

Arranging Psychophysical Colors into a Color Space

A color model is a framework for identifying and organizing psychophysical colors. I’ve already mentioned one important color model, the standard RGB model (sRGB). It identifies a color by its red, green, and blue components.

The word “color space” refers the collection of colors specified by a model. For example, the sRGB model identifies 16,777,216 distinct colors; each one can be named by three numbers for its red component (ranging from 0 to 255), green component (0-255), and blue component (0-255). Thus, white is named (255, 255, 255), black is (0,0,0), the brightest yellow is (255, 255, 0), and the blue used in the circles above is (86, 128, 255).

The word “space” here is more than a metaphor. The colors within the sRGB color space can be arranged as locations in a cube when their red, green, and blue components are marked along X, Y, and Z axes respectively.

Why would anyone want to organize color with a mathematical model and define a color space? The sRGB model was originally developed to specify all the colors that television screens can reproduce with certain red, green, and blue phosphors, and the cubical organization of sRGB colors records some important relationships among them.

Some other popular color models, like the ones produced by CIE, the Munsell Color Foundation, and the Scandinavian Colour Institute, define colors with other dimensions. They better imitate how human beings commonly distinguish among colors.

Continue reading the next post, “Speaking of Color—Part 2,” to explore some of those other dimensions of color.

***

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