Dr Moïse Tourabaly · Key points
A single pigment, melanin, brown-black in everyone. What differs from one person to another is the amount deposited in the stroma, the front layer of the iris, and the way light scatters through it.
Little melanin. Light entering the stroma is scattered by the collagen fibres, and it is the short wavelengths, blue, that come back out: this is the Tyndall effect, the same phenomenon that makes the sky blue. There is no blue pigment in the eye.
A moderate amount. The pigment's yellow-brown blends with the blue of the scattering, hence green. Hazel mixes brown and green areas within the same iris, and seems to vary with lighting and pupil size.
A lot of melanin. It absorbs most of the light: the iris looks brown, from the lightest shade to almost black. Grey eyes resemble blue ones, with more uniform scattering.
Everyone has roughly the same number of melanocytes in the iris; what differs is the amount of melanin they produce, set by several genes. The pigmented epithelium, at the back, stays dark whatever the visible colour: it stops light passing through the iris anywhere other than the pupil. In infants, the shade settles over the first months.
Educational diagram · Dr Moïse Tourabaly, ophthalmologist. It does not replace a consultation.