CMYK color model

C = cyan;
M = magenta;
Y = yellow;
K = key (black).

The mixture of ideal CMY colors is subtractive (cyan, magenta, and yellow printed together on white result in black). CMYK works through light absorption. The colors that are seen are from the part of light that is not absorbed. In CMYK, magenta plus yellow produces red, magenta plus cyan makes blue and cyan plus yellow generates green.

For several reasons, the 'black' generated by mixing the subtractive primaries is not ideal and so four-color printing uses black ink in addition to the subtractive primaries yellow, magenta, and cyan. The reasons for using black ink include:

A mixture of practical cyan, magenta, and yellow pigments is not pure black, but a dark murky color;
Mixing all three process color inks together merely to make black can make the paper rather wet, which is an issue in high speed printing where the paper must dry extremely rapidly to avoid marking the next sheet, and poor quality paper such as newsprint may break if it becomes too wet;
Text is typically printed in black and includes fine detail (such as serifs); so to reproduce text using three inks without slight blurring would require impractically accurate registration (i.e. all three images would need to be positioned extremely precisely);
Using a unit amount of black ink rather than three unit amounts of the process color inks can lead to significant cost savings, especially because black ink can be less expensive than any of the color inks.

Use of four-color printing generates a good final printed result with greater contrast. However the color a person sees on a computer screen is often different from the color of the same object on a printout since CMYK and RGB have different gamuts. For example, pure, royal blue (rgb 0, 0, 100%) is impossible to produce in CMYK. The nearest equivalent in CMYK is a dissimilar shade of purple.

Computer (and other) screens use an RGB color space, representing colors as additive mixtures of red, green and blue light (whose sum is white light). In printed materials, we cannot directly reproduce this light combination, so computer-generated images must be converted to the CMYK equivalent in ink colors.