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.
Melanin's contribution to colour is read from how tissue absorbs light. In skin it is the main absorber of visible light in the outer layer, the epidermis, while blood pigments dominate deeper down and scattering by collagen decides how far light penetrates. The appearance therefore depends on where the melanin sits as well as how much there is. In hair, the balance of two forms sets the colour: eumelanin gives black and brown, pheomelanin gives red. In the iris, the amount of melanin works together with scattering in the tissue in front of it.
A foundation or a prosthetic is matched to skin under shop lighting.
Skin's reflectance curve, shaped by melanin, blood and scattering together, is unlike that of a simple pigment mixture. A product that matches under one light can separate from the skin under another, so the match needs checking in daylight.
A camera or an image pipeline is tuned on a narrow range of skin tones.
Exposure and white-balance choices that flatter light skin can underexpose or shift darker skin. Test images need to span the range of people who will be photographed.