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Melanin -- made up of melanocyte cells -- is naturally dark brown in color but has the ability to absorb different amounts of light, depending on how much of it there is. The more melanin inside the iris, the more light is absorbed, meaning less light is reflected out, leaving the iris appearing brown.
But when someone has blue eyes, they have less melanin in their iris, resulting in less light being absorbed and more light reflecting, or scattering, back out. When this light is scattered, it reflects at shorter wavelengths along the blue end of the light color spectrum -- leaving you seeing blue.
Green and hazel eyes are somewhere in the middle, with differing quantities of melanin resulting in different levels of light absorption and therefore different colors reflecting out. Hazel is considered a mixture of eye colors, according to Heiting.
Different light settings can also make some eyes appear to change color depending on where the person is standing.
A baby's eyes can appear blue while their melanin is still forming.
"It's an interaction between the amount of melanin and the architecture of the iris itself," added Heiting. "It's a very complex architecture." This part of the eye is therefore unique to most individuals and can act as something like a fingerprint, due to the existence of various textures and patterns.
Changes In Eye Color
The iris is a muscle that expands and contracts to control pupil size. The pupil enlarges in dimmer lighting and grows smaller in brighter lighting. The pupil also shrinks when you focus on near objects, such as a book you are reading.
When the pupil size changes, the pigments in the iris compress or spread apart, changing the eye color a bit.