Color reproduction is based on the theory of three color vision. White light, which contains all the wavelengths of visible light, has three primary colors: red, green, and blue (RGB). These three colors are called  additive primaries because three lights of these three colors, when added together, produce white light.

The process of color separation is analogous to the process of seeing by the eye. The original artwork is photographed using three filters, each corresponding to one of the additive primaries.

A red filter over the lens produces a negative of all the red light of the subject. When a positive print is made, you will be left with blue and green areas. The negative has subtracted the red light from the scene and left you with cyan.

A green filter produces a positive of the other additive colors, red and blue, leaving you with magenta. A blue filter leaves red and green which produce a yellow positive.

Cyan, magenta, and yellow are subtractive primaries because each represents two additive primaries after one has been subtracted from white light. These are the colors used for process color reproduction.

When these three colors are combined in printing, the result should be a reasonable reproduction of the original. It is not. The colors are dirty and muddied. This is not due to a flaw in theory, but due to the pigmentation of the inks. A fourth color, black, is added to the mix to overcome this, improving the shadow and contrast of the image.

A halftone, or screened image created to simulate a continuos tone, is created for each color. Each of these images is printed on the press in succession creating a complete, full color image.