Colour is very complex. Basically, light is from a spectrum (continuum), but we typically manipulate colors by manipulating discrete things like pens, or the coloured dots of phosphor on a CRT which emit light of a given intensity but at a single colour, when hit by an electron of a given energy. There are several ways of mixing discrete colors to get a new colour that has the right appearance to the human eye. The human eye does not perceive a spectrum, but rather all colors as combinations of 3 so called primary colors, Red (435nm), Green (546nm) and Blue (700nm).
These primaries can be added to produce secondaries, magenta, cyan and yellow. [The roles of primary and secondary are reversed in pigments, over those in light, since the concern of a dye maker is concerned with which colour is absorbed, rather than which is transmitted]. Most MM users use colour far too much, however, in natural situations, it is very rich - this is a strange paradox.