Colour reproduction
The Young-Helmholtz three colour theory is the basis of all colour reproduction. Although demonstrated by every printed or projected colour reproduction, it remains a theory that cannot be proven.
Originally postulated in the 19th century the theory proposed that all colours could be reproduced by mixtures of the primary colours: red, green, & blue. All printed colour material makes use of the theory although implementations may differ and often utilise more colours, especially black which does not reproduce well as a mixture of other colours (this is a limitation of colour pigments, not the theory).
RGB / CMY
Red, Green, & Blue are the additive primary colours since when projected together at equal intensity they make white light. Cyan, Magenta, & Yellow are the subtractive colours since when filtered from white light, in theory, no light is left (black).
Red / Cyan, Green / Magenta, Blue / Yellow, are complementary pairs since -red is cyan, +cyan is red, and so on. Most commercial colour printers have correction keys marked CMY and most amateur enlargers are marked RGB. To any experienced operator the difference is immaterial.
Colour film
All colour film has at least three layers for the different colours, some modern film has four layers which compensates for the less than ideal pigments used. Colour negative film also has the familiar orange brown base colour which is an offset to compensate for pigment shortcomings.
Colour paper also has three layers and any colour printer must have at least three colours, almost always four including black, and sometimes as many as seven.
