Provide an overview of the trichromatic and opponent process theories of colour vision, and resolve the "debate" between the two.

The trichromatic theory proposes that the eye has three types of receptors, each responsive to one of the three primary colours of light: red, blue, and green. The eye then additively mixes different proportions of these three colours to produce the colours we see. The opponent process theory proposes that colour is signalled in pairs by receptors that fire faster to one colour and slower to a second, complementary colour. The three pairs of opponent colours are red-green, blue-yellow, and black-white.
Both theories are needed to adequately explain colour vision. In the earliest stage of information processing, there are three types of cones, each responsive to a different band of wavelengths, consistent with trichromatic theory. In later stages, cells in the retina, the lateral geniculate nucleus, and the visual cortex respond in opposite ways to complementary colours. Thus, colour coding begins with a trichromatic process and then switches to an opponent process.