four-colour process

Colour printing using four printing plates, based on the principle that any colour is made up of differing proportions of the primary colours blue, red, and yellow. Ink colours complementary to those represented on the plates are used for printing – cyan for the blue plate, magenta for the red, and yellow for the yellow. The first stage in preparing a colour picture for printing is to produce separate films, one each for the cyan, magenta, and yellow respectively in the picture (colour separations). From these separations three printing plates are made, with a fourth plate for black. The black is used for shading or outlines and type, and also to darken colour without making the ink too dense.