Unfortunately, the red, green, and blue additive primaries do not apply to every situation. To explain how printing inks produce different colors, you have to consider a different set of primary colors: cyan, magenta, and yellow (CMY). When combined in equal amounts, these three colors produce black, and are known as subtractive primaries (in printing, these colors are known as CMYK; the “K” stands for black to avoid confusion with blue, although originally “K” stood for “key plate”). If you look at the color wheel diagram, you will see that this is simply a reversal of the primary and secondary colors.

Because cyan and magenta are not often encountered in their pure form in nature, subtractive color is best considered as a technical aspect of printing, rather than as a color theory for explaining color combinations. We will investigate it further when we discuss the process of color printing and when preparing an image for print in Photoshop.

Another set of subtractive primary colors is yellow, red, and blue (YRB). This system is based on pigments and is therefore of most importance to painters. These three colors are those that aren’t created by mixing any other colors, so are the primary colors. This produces a color wheel that varies slightly to the RGB model. But the colors are essentially in the same order around the wheel. So, despite the differences, you can apply both in a similar way.

By Chris Rutter, “Color Theory Fundaementals for Digital Photography”