Opsin

Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom. 
They change from a resting state to a signalling state by absorbing light. This activates the G protein, resulting in a signalling cascade which produces physiological responses. 
This process of capturing a photon and transforming it into a physiological response is known as phototransduction.
Five groups of opsins are involved in vision. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex, but not in image-forming.
Some opsins allows vision in just a short stretch of wavelength. This is equivalent to seeing in only one colour. Two opsins allow vision in two colours, and is usual in mammals. Four opsins allows vision in full colour, and is usual in teleost fish, reptiles and birds. In mammals, only Old World monkeys, apes and humans have trichromacy, full colour vision. 
It is thought – see evolution of colour vision – that mammals lost much of their colour vision capability during the long period in the Mesozoic when they lived as mostly nocturnal animals.