Asked by: Kim Johnson, Lincolnshire
As with our skin, eye colour is determined by pigmentation. The iris, located behind the cornea (the eye's outermost layer), is itself made up of multiple layers. The outer two are collectively known as the anterior border. This contains pigment-producing cells called melanocytes. Everybody has roughly the same amount of melanocytes but our genes determine how much of a pigment, called melanin, they produce. It's the same pigment that determines skin colour and the more we produce, the darker the colour.
So a person with dark skin also has darker, browner eyes. Fairer skinned people produce less melanin. In their eyes, light passes through the anterior border and interacts with grey cells further into the iris. The light reflected back gives the eyes a bluer colour.