The following was written by Bill Evans and published in the Coolum and North Shore News, Ask the Professionals page on Friday, 25th May 2012.
Q: Why do we have different coloured eyes?
A: Our eye colour is determined by our genetics, and is a result of the amount of melanin in our irides. In general, people with darker skin have more melanin in their skin as well as in their irides and they will have darker coloured eyes. People with dark brown eyes have a high density of melanin pigment in their iris stroma (the body of their iris), as well as in the iris pigment epithelium at the back of their iris. Lighter brown eyes are the result of lower density of melanin pigment. Green or hazel eyes come from the iris stroma scattering certain wavelengths of light together with a lower density of pigment than brown eyes. Blue eyes are also caused by the scatter of certain wavelengths of light, but with much lower levels of melanin in the irides. Many babies have dark blue eyes as their body has not yet started to produce melanin, and it may take some time before the true colour of their eyes are known.