Why do human eyes have different colors?

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Diagram of a horizontal section of the human eye showing labeled parts including the cornea, iris, lens, retina, sclera, optic nerve, and blood vessels, with internal structures and muscles visible. — human eye A horizontal cross section of the human eye, showing the major parts of the eye, including the protective covering of the cornea over the front of the eye.

Human eye color is a heritable trait determined by the amount and distribution of the pigment melanin in the iris, along with light scattering. Brown eyes are the most common globally, due to higher melanin content reflecting more light wavelengths. Blue eyes result from less melanin and the scattering of shorter, blue wavelengths by collagen fibers in the iris. Green and hazel eyes are due to varying mixtures of melanin types and amounts. While once thought to be simple Mendelian inheritance, multiple genes influence eye color, making it a complex trait.1