Higher levels of melanin can have a beneficial function in intense sunlight – as in the skin, the pigment offers protection from sun damage.
In irises with little melanin, the blue color comes from the way the fibers of collagen at the back of the iris scatter light, in the same way that the sky appears blue because of way light is scattered in the atmosphere.
As to why some children’s eyes express more melanin over time, this remains mystery, says Mackey.
“We actually don’t know what influences those color changes,” says Mackey, but there could well be an environmental factor at play. “You can almost say that for everything there’s an interaction of genetics and environment, even for things we think of as totally genetic or as totally environmental,” says Mackey. “But what environmental factors could influence it? We don’t really have that data for the general population.”
While many of the changes in eye color are harmless, they can also be linked to something more serious – such as injury, infection or sun damage.
The one of the best-known eye changes from injury was David Bowie’s left eye. The striking difference between his dark left eye and pale blue right eye was the result of a punch to the head that his left pupil permanently dilated, a condition known as anisocoria. However, the punch didn’t change the blue color of Bowie’s irises, it was the enlarged pupil that made his left eye look darker.
It is, however, possible for injury to alter iris colour, says Mackey. “That can happen – if you get a lot of blood inside the eye that can stain parts of the eye. Or you can just have all the pigment scrambled everywhere and it settles.”
More commonly, infection is the root cause. A famous case of true heterochromia, where the color of the irises differs, is the actress Mila Kunis, whose right eye is brown and left green. Kunis’ heterochromia resulted from an infection of the iris, which destroyed some of the pigment in her left eye.
“Some infectious diseases can cause the pigment to disappear,” says Mackey. One is Fuch’s heterochromic cyclitis, which is caused by a viral infection – often rubella, also known as German measles. “The virus likes living in the eye, and that can flare up in later life and cause you to lose pigmentation there.”
Other viruses, too, can thrive in the interior of the eye and sometimes affect pigmentation. In one extremely rare case, an Ebola survivor experienced a change in eye color from blue to green when the virus was found to persist in his eye fluid after it had been cleared from elsewhere in his body.