Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.

In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that matching the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors.

Tetrachromacy is demonstrated among several species of bird, fish, amphibian, reptile, insect, and some mammals. It was the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.

The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells, which are lower-intensity light receptors) with different absorption spectra. This means that the organism may see wavelengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a normal human, appear to be identical. Species with tetrachromatic color vision may have an unknown physiological advantage over rival species.

In humans, preliminary visual processing occurs in the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just combine it in with an existing channel. Visual information leaves the eye by way of the optic nerve; it is not known whether the optic nerve has the spare capacity to handle a new color channel. A variety of final image processing takes place in the brain; it is not known how the various areas of the brain would respond if presented with a new color channel.

Although many birds are tetrachromats with a fourth color in the ultraviolet, humans cannot see ultraviolet light directly because the lens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. The photoreceptor cells of the retina are sensitive to near ultraviolet light, and people lacking a lens (a condition known as aphakia) see near ultraviolet light (down to 300 nm) as whitish blue, or for some wavelengths, whitish violet, probably because all three types of cones are roughly equally sensitive to ultraviolet light; however, blue cone cells are slightly more sensitive. Tetrachromacy may also enhance vision in dim lighting, or when looking at a screen.

A test for tetrachromacy is to see how many colors you see in the above image.

If you see less than 20 colors, you are like 25 percent of the population and dichromat. Dichromats have two types of color receptors.

If you see between 20 and 32 colors, you have three types of color receptors. About 50 percent of the population are trichromats.

If you see between 33 and 39 colors, you are a tetrachromat and have four types of cones. Only about 8 percent of the population is tetrachromat.