According to The Tech, scientists aren’t sure what causes gray eyes, but there are a couple of theories.
First, we have to look at the anatomy of the iris, the part of the eye that has color. There are two layers of the iris, and between these two layers is the stroma. In dark eyes, melanin (the pigment that gives eyes their color) is plentiful in the front and back layers of the iris. This melanin then absorbs any light that hits it. In lighter eyes, there is less melanin, so the light goes through the front layer of the iris, reflecting off the melanin in the back of the eye. When it hits the stroma, which is filled with collagen, the light bends and gives off a blueish hue.
Scientists think that dark gray eyes might be the result of a thin layer of melanin in the front layer of the iris, causing a sort of cloud in front of the bent light which then dims the blue color. They also think that light gray eyes might be caused by eyes having very little melanin in the front layer of the iris.