Melanin. The amount and type of pigment in the iris of the eye gives the eye it's color.
Originally, everyone had brown eyes. The brown was caused by melanin in the eye. Melanin in the eye protects the eye from the damaging effects of ultraviolet light.
melanin v
Melanin.
Melanin.
The "nucleus" of the eye that you're referring to is called the iris. Its color depends on the amount of melanin you have. The lighter your eyes are, the less melanin you have. The darker your irises are, the more melanin you have.
The level of melanin in the eye changes.
Melanin.
Melanin.
Melanin.
melanin
Melanin.