CMYK stands for cyan, magenta, yellow and key. Key means black. These are the four ink colors used in most color printing, which is often called four-color process printing.
CMYK is also the name of the color model based on these inks. It is subtractive: each ink absorbs some colors of light, and the paper reflects the rest. Screens use the additive RGB model instead. Because CMYK can usually reproduce fewer colors than RGB, photos are often converted and checked before printing. See also gamut and color space.
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