The Science Behind Eye Color

Find out what causes different colors and what it says about your health.
Person looking at self in mirror

Our eyes are made up of a complex group of features that allow us to see easily in a variety of environments. Arguably, one of the most amazing features of our eyes is their striking color.

The colored part of the eye is called the iris. The iris is responsible for controlling how much light enters the eye, which comes in through the pupil—the dark center in the middle of your iris. The iris and pupil work together to make sure the part of your brain that’s responsible for vision gets the information it needs so you can see.

What gives eyes their color

Melanin, the same pigment that’s responsible for giving your hair and skin their color, is also responsible for your eye color. And it can be different shades of black, brown, red and yellow. But what’s more influential in determining your eye color is how much melanin it has. The more melanin, the darker the eyes. For example, blue eyes don’t have much melanin, while dark brown eyes have the most.

Eye colors can be anywhere from light blue to dark brown. While you’ll commonly hear brown, green and blue eyes cited as possible colors, it’s important to note there’s a lot more variation than that. There are different shades of each eye color. For example, ‘brown eyes’ could actually be dark brown, medium brown, honey brown or amber.

Brown is the most common eye color in the world. In the U.S., about 40% of Americans have brown eyes, while only about 9% have green eyes—the rarest.

It’s not always one color

Some people have a condition called heterochromia, which is when your eyes are more than one color. Most often, heterochromia is harmless and caused by how genetics form your eyes. However, it is possible to develop heterochromia due to some medical conditions, including eye injury and glaucoma. Heterochromia affects fewer than 1% of the world’s population.

Some infants can be born with a light eye color that darkens over time. This is caused by the melanin that continues to develop after the baby is born and through childhood. In most cases, this process stops once they reach six years old.

If you look at anyone’s eye close enough, you may notice some color variation or patterns. This is normal and caused by the different tissues that make up the iris.

What do your eyes say about you? It’s not just your irises, your entire eyes reveal a lot about your overall health. That’s why routine vision care exams are important. Don’t wait—use our Find a Provider tool to find an in-network optometrist near you.


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