how is eye colour made? And why are they different colours?

“How is eye colour made? And why are they different colours?” – Jane, age 12, from Pascoe Vale South

Hi Jane,

Eyes are so fascinating and I’m glad you asked this question!

When we talk about eye colour, we’re talking about the iris or coloured area around the dark dot (pupil) in the centre of the eye. Like our fingerprints, iris colours are unique to each person.

The most common eye colour is brown, then blue, and less often green or hazel. Pigment means a substance that adds colour. The amazing thing is the human body only makes brown iris pigment (melanin) but not blue, green or hazel iris pigments. So how come everyone doesn’t have brown irises?

Read more: Curious Kids: what are the main factors in forming someone's personality?

‘What colour are your eyes?’

Iris colours can be brown, blue or green, or mixtures such as brown-yellow, greenish brown or blue-brown. The word “iris” comes from the Greek word meaning “rainbow”. In ancient Greek stories, a goddess called Iris carries messages across a rainbow bridge between Earth and the supernatural world.

Genetics – how physical traits and characteristics pass from one generation to the next – play a part in determining eye colour. In many cases, the genes that produce brown eyes are dominant, but how eye colour genes are passed on is complex. This can mean if one biological parent has brown eyes and another has blue eyes, their child is more likely to have brown eyes. But not always.

Read more: Curious Kids: why do we see different colours when we close our eyes?

But what about all the other colours?

So what is the iris made of? Our iris is inside the eye, behind a clear layer called the cornea. It’s circular and very thin (less than half a millimetre) and shaped like a donut with a hole in the middle for our pupil. The iris contains many cells, special muscles, blood vessels and nerves, surrounded by a gel material with millions of tiny crisscrossed fibres.

Iris pigment cells – melanocytes – contain pigment particles (melanosomes). Pigment cell numbers for all iris colours are about the same. But pigment particles inside the cells are different. For example, a blue iris does not have as many pigment particles as a brown iris.

The other iris cells make the tiny fibres and gel material in the iris, and other cells help protect the iris from damage. Special iris muscles with thin stretchy fibres can bunch up or relax the iris to control our pupil size in bright or dim light.

The back of the iris has a dark brown surface because of cells filled with brown pigment. This back surface pigment helps our vision as it stops light scattering through the iris.