The CMYK system
In these illustrations, white light is shown as having three components corresponding to the additive primaries: red, green and blue. All of these are reflected from white paper. The printers inks are shown as thin layers, which act as subtractive filters. Other colours are produced by varying the quantities if the cyan, magenta a yellow inks. For example, if a patch of yellow is partially covered by tiny magenta dots, the mixture of yellow and red will appear to be orange from a normal viewing distance. (This simplified explanation works surprisingly well for carefully selected printing inks. The complications glossed over may be significant for other pigments.)
Cyan, magenta and yellow are used in this way to make photographic prints and colour photocopies; they are used for the illustrations in books and magazines; they are used in most computer printers.
Using cyan, magenta and yellow as primaries is called the CMY system for specifying colours. A CMY printer can produce a range of colours called its colour gamut, but this range is limited by the available inks. In most cases greater densities are achieved by using a black ink as well. This is called 4-colour printing. The system is then called the CMYK system.