Before moving on to weightier matters such as gravity, calculus, and his impending thirtieth birthday, Sir Isaac Newton provided the world of color with one more key concept: if you take the colors of the spectrum and arrange them around the circumference of a wheel, the relationships among primaries become much clearer (see Figure 4-2).
Figure 4-2. The color wheel
The important thing to notice about this color wheel is that the additive and subtractive primary colors are opposite each other, equidistant around the wheel. These relationships are key to understanding how color works. For instance, cyan sits opposite red on the color wheel because it is, in fact, the opposite of red: Cyan pigments appear cyan because they absorb red light and reflect blue and green. Cyan is, in short, the absence of red.
Colors that lie directly opposite each other on the wheel are known as complementary colors.
Figuring Saturation and Brightness
So far, we've talked about color in terms of three primary colors. But there are other ways of specifying color in terms of three ingredients. The most familiar one use the terms of hue (the property we refer to when we talk about "red" or "orange"), saturation (the "purity" of the color), and brightness.
Newton's basic two-dimensional color wheel lets us see the relationships between different hues, but to describe colors more fully, we need a more complex, three-dimensional model. We can find one of these in the HSB (Hue, Saturation, Brightness) color cylinder (see Figure 4-3).
Figure 4-3. The HSB color cylinder
In the HSB cylinder, you can see the hues are arranged around the edge of the wheel, and colors become progressively more pastel as we move into the centerthe farther in you go, the less saturated or pure the color is. The Apple Color Picker (which many programs use to specify color, but not Photoshop) is a graphical representation of the HSB color modelit displays a color wheel to pick hue and saturation, with a slider to control brightness.
Tristimulus Models and Color Spaces
Ignoring the inconvenience of CMYK, all the ways we've discussed of specifying and thinking about color involve three primary ingredients. Color scientists call these tristimulus models. (A color model is simply a way of thinking about color and representing it numerically: A tristimulus model represents colors by using three numbers.) If you go deep into the physiology of color, you'll find that our perceptual systems are actually wired in terms of three different responses to light that combine to produce the sensation of color. So the tristimulus approach isn't merely a mathematical convenienceit's inherent in the way our perceptual systems work.
But tristimulus models have another useful property. Because they specify everything in terms of three ingredients, you can (with very little effort) view them as three-dimensional objects with x, y, and z axes. Each color has a location in this three-dimensional object, specified by the three values. These three-dimensional models are called color spaces, a term that gets thrown around a great deal in the world of color.
We like to think of the HSB color space as a giant cylinder; the brightness slider in the Apple Color Picker determines which slice of the cylinder we're looking at. But, like any metaphor, there's a good side and a bad side to looking at color this way.
Therefore, while the Color Picker is a step in the right direction, we have to go further to understand how to work with color.