A natural pigment responsible for the colour of skin, hair and eyes in humans, ranging in amount and type between individuals.
Melanin exists mainly as eumelanin (brown-black) and pheomelanin (red-yellow), produced and stored in cell structures called melanosomes. In the eye, the amount and distribution of melanin in the iris stroma and pigment epithelium is the main factor determining eye colour.
Melanin is not itself a 'blue' or 'green' pigment — no human eye colour comes from a blue or green pigment. Higher melanin produces browner eyes; lower melanin allows light-scattering effects (see light scattering) to produce blue, grey or green appearances instead.
Dark brown eyes have dense melanin throughout the iris; blue eyes have very little, which is why light scattering rather than pigment absorption dominates their appearance.
Melanin is the starting point for understanding eye colour without oversimplifying it into a single 'blue vs brown pigment' story.