Two different light spectra which have the same effect on the three color receptors in the human eye will be perceived as the same color.

Dogs seem to have only two kinds of color receptors in the eye.

By definition, that requires that they have color receptors containing at least two different visual pigments.

One possible reason people see colors may be that the color receptors in the human eye respond at different rates to red, green, and blue.

However, the human eye normally contains only three types of color receptors, called cone cells.

Each color receptor respond to different ranges of the color spectrum.

Humans and other species with three such types of color receptors are known as trichromats.

Human eyes have color receptors known as cone cells, of which there are three types.

For example, for species known as tetrachromats, with four different color receptors, one would use four primary colors.

Some creatures, like insects, have a fourth primary color receptor.