CMYK is a scheme for combining primary pigments. The C stands for cyan (aqua), M stands for magenta (pink), Y for yellow, and K for Key. The key color in today’s printing world is black but it has not always been. During the early days of printing, the colors used for Key have been brown, blue, or black — whichever was the cheapest ink to acquire at any given time.
The CMYK pigment model works like an “upside-down”version of the RGB (red, green, and blue) color model. Many paint and draw programs can make use of either the RGB or the CMYK model. The RGB scheme is used mainly for computer displays, while the CMYK model is used for printed color illustrations (hard copy).
There is a fundamental difference between color and pigment. Color represents energy radiated by a luminous object such as a cathode ray tube (CRT) or a light-emitting diode (LED). The primary colors are red (R), green (G), and blue(B). When you see a red area on a CRT, it looks red because it radiates a large amount of light in the red portion of the electromagnetic radiation spectrum (around 750 nanometers), and much less at other wavelength. Pigments, as opposed to colors, represent energy that is not absorbed by a substance such as ink or paint. The primary pigments are cyan (C), magenta (M), and yellow (Y). Sometimes black (K) is also considered a primary pigment, although black can be obtained by combining pure cyan, magenta, and yellow in equal and large amounts. When you see yellow ink on a page, it looks yellow because it absorbs most energy at all visible wavelengths except in the yellow portion of the spectrum (around 600 nanometers), where most of the energy is reflected.
The primary pigments and the primary colors are mathematically related. Any two pure radiant primary colors (R, G, or B), when combined,produce radiation having the appearance of one of the pure non-black primary pigments (C, M, or Y). Any two pure non-black primary pigments, when mixed, produce a substance having the appearance of one of the pure primary colors. These relationships are depicted in the illustration.
The primary colors RGB, combined at 100-percent brilliance,produce white. The primary pigments CMY, combined at maximum concentration,produce black. Shades of gray result from the equal (but not maximum) brilliance of R,G, and B, or from equal (but not maximum) concentrations of C, M, and Y. If you have a paint or draw program such as Corel DRAW! that employs both the RGB and the CMYK schemes, you can investigate these relationships by filling in regions with solid colors using one mode, and examining the equivalent in the other mode. After a while you will develop an intuitive sense of how these schemes work, how they resemble each other, and how they differ.