There is no blue pigment in blue eyes, only scattered light
Blue eyes contain no blue pigment whatsoever, and neither do green ones. Their colour comes from light bouncing around in the front layer of the iris, the same kind of scattering that makes the sky blue. That is why lighter eyes seem to shift shade as the lighting changes.
Eye colour depends on two things: how much melanin sits in the iris, and how the tissue of its front layer, the stroma, scatters light. Human irises only contain pigment ranging from light brown to black. Blue, green and hazel appear through Tyndall scattering, a relative of the Rayleigh effect in the atmosphere, making them a structural colour rather than a painted one. Hazel eyes add a yellowish pigment, lipochrome, which gives a golden or coppery tint in sunlight.
Brown is overwhelmingly the norm. About 79 percent of people have brown eyes, and almost all mammals do too; mammals are born with blue eyes and darken afterwards. Birds are a different story, using extra pigments such as carotenoids to produce vivid colours. People of European ancestry show the widest range of eye colours of any population, and a 2020 ancient-DNA study suggested the common blue-eye variant came from the Near East, reaching Europe around 42,000 years ago.
The genetics are messy. As many as 16 genes have been linked to eye colour, though two on chromosome 15 dominate. OCA2, named after a form of albinism it can cause, influences blue and green eyes along with freckles and hair tone, while a mutation in HERC2, which regulates OCA2, is partly responsible for blue eyes. Even so, a 2010 photographic study identified ten genes that together explained only about half the variation. Still, a 2009 Rotterdam study predicted brown or blue with over 90 percent accuracy from just six genetic markers.
Eye colour is not always fixed. A 1997 study of White Americans found that 17 percent of children's eyes changed by adulthood, half lightening and half darkening; green eyes tended to darken, hazel and light brown to lighten. Surprisingly, 11 percent of their mothers changed too, mostly towards lighter shades.
Source: Eye color
