In humans, the pigment of the iris varies from light brown to black. This depends on the amount of melanin in the iris. The appearance of blue, green and hazel eyes result from the Rayleigh scattering of light in the iris. A similar process accounts for the blueness of the sky. Neither blue nor green pigments are ever present in the human iris or fluid. Eye colour is an example of structural colour which varies according to the lighting conditions, especially for lighter-coloured eyes.
The brightly coloured eyes of many birds, reptiles and insects is caused by other pigments, such as
pteridines, purines, and carotenoids.