What characteristic of the iris contributes to eye color?

The characteristic of the iris that contributes to eye color is the amount of melanin present. Melanin is a pigment found in various parts of the body, including the skin, hair, and eyes. In the context of the iris, the concentration and distribution of melanin determine the coloration of the iris. For instance, a higher amount of melanin leads to darker eye colors, such as brown, while lower levels result in lighter colors, like blue or green.

The amount of melanin affects the way light is absorbed and reflected by the iris, influencing not only the color seen but also the perceived intensity of that color. Factors such as genetics play a crucial role in determining the melanin levels in a person's iris, which is why eye color can vary significantly among individuals.

Other factors, such as the amount of aqueous humor, the size of the pupil, and the shape of the cornea, do not influence eye color directly. While they can affect visual characteristics and clarity of vision, they do not alter the pigmentation or coloration of the iris itself.