The iris is the colored part of the eye. It helps control the amount of light that enters your eye. It surrounds the pupil, the dark center of your eye. The iris and pupil team up to regulate how much light focuses on the retina, which detects light and tells the brain to kick off the vision process. Variations in our genes dictate eye color. Most of those genes contribute to making, moving, and storing melanin. The amount of melanin determines your eye color. Blue and gray eyes have very little melanin, while eyes with a tad more melanin are green, hazel, or light brown. Eyes with the most melanin are amber, medium brown or dark brown.

Source: All about vision