The flat, colored, ring-shaped membrane of the eye is called the iris.
The iris is located between the cornea and the lens of the eye. It is the part of the eye that gives it its distinct color, such as blue, green, brown, or gray. The color of the iris is determined by the amount and distribution of pigmented cells within it.
One of the primary functions of the iris is to control the amount of light entering the eye. It does this by adjusting the size of the pupil, the central opening in the iris. In bright light, the iris contracts and makes the pupil smaller to reduce the amount of light entering the eye. In low light conditions, the iris expands, making the pupil larger to allow more light to enter.