The eyes’ colour is determined by the melanin content, a pigment produced by melanocytes in the eye’s iris. The "pure" eye colours are: dark brown (almost black), brown, light grey and dark blue.
Green is actually a depigmented brown due to mixtures with lighter tonalities. Pale "bright" blue eyes are a mixture between light grey and dark blue (we do not take into account the pink and red colours of people with albinism, who are not a particular race).
Two (probably more, although they are not yet known) are the genes that influence the iris pigmentation: HERC2 and OCA2. The OCA2 gene encodes/synthesises a protein that transports tyrosine (precursor of melanin) and therefore determines eye pigmentation through the content of melanin in the iris (highly active OCA2= large amounts of melanin: dark eyes. Scarcely active OCA2= small amounts of melanin: green eyes). HERC2 (a gene that regulates OCA2 activity), in its mutated form, disconnects totally the OCA2 mechanism, hindering its expression, which in turn produces the light colour due to the lack of melanin.
How does the blue colour occur? In brown eyes, light is absorbed by the melanin contained in the external layers of the iris. However, in blue eyes, the outer layers have hardly any melanin, so that the light passes directly into the inner layers. There, light is scattered by proteins in such a way that, when reflected back out of the iris, it gives a blue tonality. This is the same optical phenomenon that makes the sky look blue. All colours between brown and blue (like hazel, amber or green) are determined by the amount of melanin involved in the process as well as the number and size of the proteins inside the eye.
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