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### The Color Wheel

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 we 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 to 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 describes color in 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 Apple Color Picker (see Figure 4-3).

##### Figure 4-3. The Apple Color Picker

In the Apple Color Picker, we can see the hues are arranged around the edge of the wheel, and colors become progressively more “pastel” as we move into the center—the farther in you go, the less saturated or “pure” the color is. Beside the wheel, we have a slider that makes the color lighter or darker. The Color Picker is a graphical representation of the HSB (hue, saturation, and brightness) color model.

#### 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 is more than just a mathematical convenience—it has a solid basis in the way our nervous systems work.

But tristimulus models have another useful property. Because they specify everything in terms of three ingredients, we 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.

• The good side. The Apple Color Picker is a great way to start learning about color, and how changing a single primary changes your colors.

• The bad side. The simple HSB model can't really describe how we see colors. For instance, we know that cyan appears much lighter than blue; but in our HSB cylinder, they both have the same brightness and saturation values.

Therefore, while the color picker is a step in the right direction, we have to go further to understand how to work with color.

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