Brown eyes get their color from melanin, the same pigment that colors your skin. But blue eyes don’t have any blue pigment in them. Blue eyes get their color the same way water and the sky get their blue color. They scatter light so that more blue light reflects back out.
The colored part of the eye is called the iris. It’s a structure that contains muscle and other kinds of cells. You can see the iris in action when it squeezes or relaxes to let in more or less light through the pupil. The iris is made up of two layers. For almost everyone — even people with blue eyes — the back layer (called the pigment epithelium) has brown pigment in it.
The front layer of the iris (called the stroma) is made up of overlapping fibers and cells. For people with brown eyes, some of the cells also have brown pigment in them. If there is no pigment at all in this front layer, the fibers scatter and absorb some of the longer wavelengths of light that come in. More blue light gets back out and the eyes appear to be blue.
For people with green or hazel eyes, one or both of the layers of the iris also has some light brown pigment in it. The light brown pigment interacts with the blue light and the eye can look green or speckled. Many people have variations in the color of their irises, often with one color near the pupil and another at the edge. This variation happens when different parts of the iris have different amounts of pigment in them.
Do Eyes Change Color?
Since blue eyes get their color from the light that’s coming in and being reflected back out, they really can appear as different colors depending on the lighting conditions. Green and hazel eyes are a mixture of pigment color and color from scattered light, so they can also look different in different lighting conditions.
Babies often do not have much pigment in their irises when they are born. This is why their eyes can look very blue. More pigment accumulates in the iris over the first few months of a child’s life and blue eyes can become less blue or even turn completely brown. For most children, eye color stops changing after the first year, but for some kids the color can continue to change for several more years.
Why Do We Have Blue Eyes?
The genetics of eye color are very complicated. You can’t predict a child’s eye color just from looking at the parents’ eyes. Even parents who have the same color eyes as each other could have a child with different colored irises.
Genetic research has shown that blue eyes probably only appeared in the last 6,000 to 10,000 years. Before then, everyone had brown eyes. Blue eyes have probably spread through the population just because some people like how they look and chose to have children with blue-eyed people.