The iris is the colored portion of the eye, and is comprised of a muscular membrane that controls the amount of light entering the eye. The expanding and contracting tissue of the iris produces adjustments in the aperture of the eye, termed the pupil. If the incoming light is too bright, the iris decreases the diameter of the opening. Pupil diameter increases when more light is required. Eye color in the animal kingdom varies due to the presence or lack of pigments called melanin that are embedded in the iris. If melanin is uniformly present in the external layer of the iris, eyes will appear to be brown. If the iris lacks melanin, the eye will appear blue for the same reasons that the colorless atmosphere appears blue. Unevenly distributed deposits of melanin display a brownish green color and such eye coloring is typically referred to as hazel. Some eyes contain a yellowish pigment called carotene, which combines with blue to create shades of green. The eyes of most primates are not highly colorful and many typically have dark colored irises.