Color vision has reached a high level in primates, especially humans. While survival today may not depend on whether an individual can see colors, our ancestors who evolved to perceive a full range of colors could better spot food and predators, giving them a distinct advantage over those whose color perception was less robust. Humans can perceive colors in the millions, by some estimates up to 10 million different colors.

The human eye consists of an outer cornea, covering the iris, which expands and contracts to dilate the pupil and control the amount of light entering the eye. Beyond the iris is the lens, which refracts light and projects a reduced, inverted image onto the retina lining the back of the eye. Although the image projected onto the retina is inverted, or upside down, we have evolved to automatically compensate in our brains so that what we see corresponds with our other senses in our physical world.

The retina consists of photoreceptor cells that translate light's electromagnetic radiation into packets of nerve impulses that are sent to the brain via the optic nerve. These impulses stimulate the brain to perceive colors. The cerebral cortex, the most highly evolved part of the brain, processes visual information and combines it with memory and associations to produce each individual's unique visual sensation.

The optic disc immediately over the optic nerve channel to the brain contains no photoreceptors at all, causing a small blind spot, but our vision compensates for this since we move our eyes frequently.

The retina's photoreceptor cells consist of rods and cones. Both rods and cones contain photopigments, which generate the electrical signals sent to the brain.

Rods help us to see in scotopic low light settings, and perceive differences in lightness and darkness only, or differences in grayscale value. Rods contain a photopigment called rhodopsin, which bleaches in the presence of light, a reversible process, but one that causes temporary visual impairment known as dark adaption when entering a dark area from a light one.

Light must be present for humans to perceive color. In a completely dark setting, where no light can enter, the eye cannot perceive anything. In a darkened room, where only outlines of objects can be seen, these objects are visible because there is still a low level of light present in the room, although it may not be enough light to allow colors to be seen.

Cones in general are more sensitive to longer wavelengths, or red light, while rods are more sensitive to shorter or blue wavelengths. This means that in high light levels, a red object may appear brighter than a blue one, while in low light levels where values are perceived more than hues, the same blue object may appear to be the brighter one. This phenomenon is known as the Purkinje shift, after the Czechoslovakian physiologist Johannes Purkinje, who discovered it in the 1800's.