Color vision is the ability to distinguish among light stimuli based solely on differences in their spectra, independent of brightness, polarization, or any other stimulus feature. To make such discriminations, at least two receptor classes with different sensitivity spectra must exist in the retina. In vertebrates, this generally means that two cone spectral classes are present. While invertebrate photoreceptors vary in their cellular structure, two or more types must nevertheless be present to provide color vision. Because color vision involves internal processing to distinguish among stimuli, it cannot be demonstrated by physiological means alone. The definitive proof that an animal possesses this ability requires rigorous behavioral testing to show that it can make the required discriminations. Often, this is difficult to do, so it is not always a simple matter to show that color vision is present in a given species.