CMY and the practicalities of printing: Why do we need K?
The most common form of full-color printing is based on the clever use of red, green, and blue filters (in the form of cyan, magenta, and yellow ink, respectively) to subtract, or filter, different wavelengths from the white light reflected by the substrate. We can vary the amount of light filtered by each ink by allowing some of the background (substrate) to show through unfiltered. This is called ‘screening.’ A screened area with a uniform percentage of ink (for example a patch screened so that it is 70% cyan ink and 30% paper) is called a tint. Theoretically, when you combine equal tints of cyan, magenta, and yellow, you should get a neutral shade of gray,* and when all are at 100% ink and 0% paper, you should see black. However, commercial inks and papers are far from perfect, and when inks are printed on top of one other, they don’t always behave in an ideal way. It is practically impossible, for example, to manufacture a cyan ink that filters out only red and absolutely no green or blue. The result is that when you print a patch that is 100% cyan, magenta, and yellow, you don’t get a pure black. Instead, you get a muddy, dark brown patch of oversaturated ink that can cause drying issues and printed sheets that stick together. To get better blacks and grays (including the black ink needed for text), printers reduce the overall amounts of the CMY primaries and add quantities of black ink. “CMYB” would be a confusing name for this process as the letter B is frequently used to denote “blue.” So printers use the letter K, for “key,” to give us “CMYK,” also commonly referred to as four-color printing.
Printers like the fact that CMYK uses less ink, saves money, and shortens drying time. But CMYK is just one form of process color printing, a general term for any mechanism that generates colors using quantities of primary inks. In fact, there are some process-color systems that use up to seven or more primary inks.
It is useful to think of RGB and CMY as simple transformations of each other. In fact, CMY can be thought of as a special form of RGB—one that uses negative quantities of red, green, and blue. The key idea to remember with these two color spaces is that three primary colors, combined in different ways, are all that’s needed to fool the human eye into thinking it’s seeing all possible colors.
* In a perfect world, equal percentage tints of cyan, magenta, and yellow would produce a neutral gray. In reality, this is not the case because inks, substrates, and printing processes are far from ideal. Therefore, a neutral gray is never achieved by printing equal values of yellow, magenta, and cyan.
