Human retinas have four types of light receptor cells. The highly sensitive 'Rod' cells are active in low light conditions, but don't contribute at all to color vision.

There are also three* ^{[see note below]} different types of 'Cone' cells which respond differently to coloured light (i.e. different wavelengths).

It has now been established, however, that that colour vision is not exclusively determined and processed in the retina. The brain is also involved in colour processing.

It is now abundantly clear that colors are not determined in the retina alone, but there are additional processes in the brain shaping the relationship between stimulus and perceptual experience, the process details and location in the brain often as yet unknown."

Source :

It's been found that damage to various parts of the brain can lead to complete colour blindness, even though the retina is fully functional. See: Cerebral achromatopsia at Wikipedia.

Although many discrete brain areas have been implicated, the neural processing which leads to colour vision in currently unknown.

Despite all the debate about where in the brain color constancy is achieved, the physiological mechanisms of how we achieve color constancy are little understood.

Source :The Visual Neurosciences MIT books

* Note: Many animals have more that three types of colour receptor cell, giving them the ability to see a greater colour range than humans. Several species of fish, for example, can see well into the ultra-violet range (≈360 nm). Recent research is suggesting the some humans may have remnants of a fourth type of cell. So-called 'Tetrachromacy.'

In 2010, visual tests on one US patient found the presence of extra cone cells, along with significantly enhanced colour vision in some colour ranges - but less than normal in others. See : Journal of Vision Vol.10, 12.

Also see : Photoisomerizationplugin-autotooltip__plain plugin-autotooltip_bigPhotoisomerization

Photo-isomerization is the name given to the structural re-arrangement of asymmetric chemical compound molecules from one 'isomer' (e.g. 'left-handed' or 'right handed') to another, when illuminated.

The process is normally reversible, is dependent on the wavelength ('colour') of the light, and is extremely rapid, in the order of picoseconds.. The process by which photons flip the structure of the Rod and Cone cells' photodetector compound Retinal is not yet fully understood.