As it turns out, blue colored eyes aren’t actually blue. Eyes with these colors contain no pigment, rather their color is derived entirely from the eyes structural network of muscles.
The colored part of eyes is called the iris, which is made up of two different parts: the epithelium and the stroma. The epithelium is only a mere 2 cells thick and is where brown and black pigments are stored. If you look closely at many people’s eyes, you’ll notice small black specks in their iris, these spots are a result of pigment in the epithelium.
The stroma, on the other hand, is made up entirely of colorless collagen. On occasion, the stroma will contain melanin, but this doesn’t occur in every eye.
These two different parts of the iris acting together control what eye color you have.
Brown eyes contain a high concentration of melanin in their stroma which absorbs light giving the eye a darker color. Green eyes don’t contain much melanin in their iris.
This ultimately means that while some light is absorbed by pigment, the particles in the stroma of people with green eyes can scatter light in what is called the Tyndall effect, creating a blue hue.
When you combine this blue hue with brown melanin, eyes will appear green.
More fascinating than how green eyes work, however, is how blue eyes get their color. They have a completely colorless stroma with no pigment at all and they also have no collagen. All this science means that as light enters the iris, it is scattered back into the atmosphere and along with the Tyndall effect, creating a blue hue. Since blue-eyed people don’t have brown melanin to turn that blue hue green, their eyes appear blue.
Ultimately, blue-eyed people don’t have any pigment in their eyes and their eyes, in turn, actually don’t have a color. The blue appearance is a result of how light reflects off the iris. This is a similar principle to how some butterflies and berries get their color as well.
Source: Science Alert