The red green and blue (RGB) colours are the primary constituent parts of white light. These colours are called additive primaries because, when added together, they produced white light. Cyan, magenta dan yellow are called the subtractive primaries and are used in the four-colour printing process.
Colour reproduction is based on the same principles as the three colour vision of the human eye. The eye contains three different types of receptors, each of which are sensitive to one of the primary RGB colours of light. Any two additive primaries will create one of subtractive primaries (as can be seen where the colours overlap in the diagram above). Similarly, any two subtractive primaries create an additive primary. This is the principle behind the separation process used to reproduce colour images.