CMYK color mode is used as an overprinting model, using the three primary colors principium to form a variety of complex colors by the mixture and superposition of four colors, the four standard colors are
C= cyan, M= magenta, Y=yellow, K= black
Magenta plus yellow will be red; magenta plus cyan will be blue; cyan plus yellow will be green; Theoretically, only the superposition of the three colors mentioned above would be enough in forming black, but in actual printing, the superposition of the three colors can only create a dark gray or dark brown (because of the impurities in the ink). Moreover, three-layer color is not able to become dry immediately, thus is not conducive to quick printing;
Precise overprinting is needed in three-layer printing, replacing the three-layer printing with black color can significantly cut costs. Thus four colors’ matching method was adopted. Black is used as the color that locates the position of registering in printing, so it is also called key registering color (Key), using K rather than B to represent the black to avoid confusing with blue (B).
The amount of other colors substituted with black depends on different printing technology, paper and quality of black ink. Since printing and computer display use different color modes, computer tends to use RGB mode, thus the image displayed on the computer screen has some differences in colors with the print-out copy.
Color mode conversion
This conversion is not fully consistent, for example, RGB model can be converted to printin g mode, and printing materials can still be converted to the RGB mode. However, if a printing image was converted to RGB mode, and then the RGB mode image was converted back to print mode, there will be color distortion. Color differences will exist between the two printing images. Thus if precise color is required in business application, do not use the conversion method.