Eye color is a visible character caused by two factors: the pigmentation of the eye's iris and the way light is scattered as it hits the top layer of the iris. 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 genetics and inheritance of eye color in humans is complicated. So far, as many as 15 genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2. The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. However, OCA2 gene polymorphism, close to proximal 5' regulatory region, explains most human eye-color variation.