The iris sits in the eye’s front chamber between the world facing cornea and the brain facing lens.   A wonderfully flexible and responsive curtain. Effortlessly opening and closing, regulating the intensity of light to the retina.

Why blue?

Nearly all baby irises are blue and we wait to see what adult colours will come.   

It is the colour of the sky and for good reason.    

The iris must block light and this is mostly done by a double layer of cells on its lens facing rear surface.   They are heavily pigmented almost black by varieties of melanin, the same pigment that colours hair and skin. They are so dark that they hardly contribute to iris colour.

In a baby, the main iris body, the stroma, lacks pigment.    The blue is produced there, like that of the sky, by Rayleigh scattering of particles smaller than the wavelengths of visible light.    With age, melanins develop in the stroma too and in the front layer. These, combined with the blue rich Rayleigh scattering give the range of human eye colours.   

Browns, greens, grey all arise from different concentrations of basically brownish melanins and Rayleigh (perhaps sometimes also Mie) scattering from tiny stroma particles and structures.