The perceived colour of visible light is to some extent a subjective experience. The following is a potential spectral energy distribution of light reflecting from a green wall. The retina has both rods and cones , as shown below. It is the cones which are responsible for colour perception. There are three types of cones, referred to either as B, G, and R, or equivalently as S, M, and L, respectively.
Animals exist with both fewer and more types of cones.
The photopigments in rods and cones are stimulated by absorbed light, yielding a change in the cell membrane potential. Such spectra are called metamers . Our perception of colour is related only to the stimulation of three types of cones. If two different spectra stimulate the three cone types in the same way, they will be perceptually indistinguishable. Note that the phosphours of colour TVs and other CRTs do not emit pure red, green, or blue light of a single wavelength, as is the case for this experiment.