How people see colours

How do cones in the retina recognise colours?  

Cones are concentrated in the fovea at the centre of the retina.  Three populations of cones have three different variations of a protein, rhodopsin, which changes shape when hit by light of a particular frequency.  Rhodopsins combine with vitamin A and subtle variations in amino acids change their sensitivity to light.  

Each population responds to several photons of light of a particular range of wavelengths.  When the protein changes shape it blocks glutamine secretion, causing a silence. Look how spread out their responses are; the 'red' cones actually peak in the yellow/green wavelength.

Dogs have only two different populations of cones so they have red/green colour blindness. WE can't detect ultraviolet light, with wavelengths of 10-400 nm but hawks can, they have a fourth population of cones which respond to ultraviolet rays. Mouse urine and hair glows ultraviolet due to the presence of certain amino acids. 

Insect eyes have a similar sensitivity beyond human vision so bees can see 'nectar guides', ultraviolet indicators in flower petals.