The pattern of brown and red pigment, collagen fibres, and the topography of the iris determines the colour of the human eye.
Iris is the coloured area at the front of the eye.
It is about 12 millimetres in diameter and has an opening in the middle and is called the pupil.
The only pigment that contributes substantially to normal human iris colour is the dark pigment melanin.
The factor that determines the phenotypic eye colour of a person is the quantity of melanin pigment in the iris.
The amount of light entering the eye is regulated by the coloured part of the eye.
When the light is bright outside, the iris closes the pupil which allow less light to enter. And when the light is dull outside, the iris opens up the pupil to allow more light to enter inside.