The concept of primary colors is at the heart of much of the color work we do on computers. When we work with primary colors, we're talking about three colors that we can combine to make all the other colors. We can define colors by specifying varying proportions of primary colors, and we can color-correct images by adjusting the relationship of the primary colors. Ignoring for the moment which specific colors constitute the primaries, there are two fundamental principles of primary colors.
The secondary colors, by the way, are made by combining two primary colors and excluding the third. But we don't much care about that. It is important to note, though, that what makes the primary colors specialindeed what makes them primary colorsis human physiology rather than any special property inherent in those wavelengths of light.
Additive and Subtractive Color
Before becoming preoccupied with the behavior of spherical objects like apples, billiard balls, and planets, Sir Isaac Newton performed some experiments with light and prisms. He found that he could break white light down into red, green, and blue components, a fairly trivial phenomenon that had been known for centuries. His breakthrough was the discovery that he could reconstitute white light by recombining those red, green, and blue components. Red, green, and bluethe primary colors of lightare known as the additive primary colors because as you add color, the result becomes more white (the absence of colored light is black; see Figure 4-1). This is how computer monitors and televisions produce color.
Figure 4-1. Additive and subtractive primaries
But color on the printed page works differently. Unlike a television, the page doesn't emit light; it just reflects whatever light hits it. To produce color images in print, you don't work with the light directly. Instead, you use pigments (like ink, dye, toner, or wax) that absorb some colors of light and reflect others.
The primary colors of pigments are cyan, yellow, and magenta. We call these the subtractive primary colors because as you add pigments to a white page, they subtract (absorb) more light, and the reflected color becomes darker. (We sometimes find it easier to remember: You add additive colors to get white, and you subtract subtractive colors to get white.) Cyan absorbs all the red light, magenta absorbs all the green light, and yellow absorbs all the blue light. If you add the maximum intensities of cyan, magenta, and yellow, you get blackin theory (see Figure 4-1).
Mrs. Anderson had the right idea about primary colors; she just picked the wrong ones. No matter how hard you try, you'll never be able to make cyan using red, yellow, and blue crayons.
An Imperfect World
A little while ago, we asked you to trust us on the subject of CMYK. Well, we just told you that combining cyan, magenta, and yellow would, in theory, produce black. In practice, however, it produces a muddy brown mess. Why? In the words of our friend and colleague Bob Schaffel, "God made RGB . . . man made CMYK." To that we add: "Who do you trust more?"
If we had perfect CMY pigments, we wouldn't have to add black (K) as a fourth color. But despite our best efforts, our cyan pigments always contain a little red, our magentas always contain a little green, and our yellows always contain a trace of blue. Moreover, there's a limit to the amount of ink we can apply to the paper without dissolving it. So when we print in color, we add black to help with the reproduction of dark colors and to achieve acceptable density on press. See Chapter 5, Color Settings, for more on this.
If we only had to deal with CMY, life would be a lot simpler. However, a large part of the problem of reproducing color images in print is that scannerssince they deal with lightsee color in RGB, and we have to translate those values into CMYK to print them. Unfortunately, this conversion is a thorny one (see Chapter 5, Color Settings, for more on this subject).