An individual who has two photopigments is called a dichromat. The absorption spectra of the two photopigments (P1 and P2) peak at different wavelengths but overlap over a considerable portion of the spectrum. If the subject is shown two patches of light, one patch consisting of A and the other B, it is now impossible to adjust the intensity of B so that it will match A. This is because the number of quanta absorbed by the two photopigments for A never equals the number absorbed by these same photopigments for B. No matter how the intensity of A and B are changed, they never have identical effects on both photopigments. They are therefore distinguishable solely on the basis of their wavelengths.
Now consider the situation shown below.
Here we have two patches of light; one consisting of A and the other of a mixture of B and C. The subject is allowed to control the intensity of each wavelength independently. Examination of the graph shows that it is possible to adjust the intensities such that each patch stimulates both photoreceptors by the same amount. Under these conditions the two patches of light will look identical, despite the fact that physically they are quite different.
Such stimuli are referred to as metamers.
From the above discussion it can be concluded that a monochromat is color blind and can match any wavelength with any other wavelength. A dichromat is able to discriminate between a limited range of wavelengths and can match any wavelength using a mixture of two other wavelengths.