A person with only one photopigment is known as a monochromat.
The graph above shows an absorption spectrum for a hypothetical photopigment. A quantum of light with a wavelength of A has the highest probability of being absorbed by the photopigment, whereas a quantum with a wavelength of B has half the probability of being absorbed. Although A is twice as likely to be absorbed than B, the photopigment has no way of knowing which wavelength it absorbed. For a given amount of light energy, A would look brighter than light B. However, the subject would have no way of telling if the difference in brightness was due to a difference in intensity or a difference in wavelength. Furthermore, if the intensity of A was doubled so that it matched the brightness of B, the subject would be unable to differentiate between the two wavelengths. Therefore a monochromat is unable to distinguish between objects on the basis of wavelength alone and can be considered to be color blind.