There is a round hole in the middle of iris, called pupil. Light enters the eye through the pupil. In order to make objects appear more clearly on the retina, our eyes are like a black box, so the pupils generally look black. Iris is rich in pigment cells and blood vessels, and the color of iris is determined by the melanin content in pigment cells. The more melanin, the darker the iris color, the less melanin and the lighter the iris color.
Common iris colors are dark brown, light brown, grayish white, blue, green and so on. Different races have different iris color distributions. The common dark brown, light brown and grayish white irises of yellow people and black people are all caused by different melanin contents. The reason why the blue or green iris is blue or green is that it contains little melanin and is light in color, coupled with light scattering and the adjustment of microvascular color. Irises of this color are mainly distributed among white people in Europe and America.
If the patient is albino and the iris has no melanin, the iris will show the color of microvascular and pink. The color of iris is determined by the concentration of melanin, and the amount of melanin is controlled by genes, so individuals of different races and the same race may have different eye colors.