The Iris is donut-shaped, with the hole being the Pupil of the eye. A lot of people are surprised to find out that the pupil is a hole and not a black spot. Our light ray will not touch the Iris, but go past it, through the pupil. The color of the Iris is a function of the amount of pigment deposited on its surface. The natural color of all Irises is blue.
Green eyes have just enough brown pigment mixed in with the blue to give the appearance of green.
The more brown the eye appears, the more pigment has been laid down on the iris. The Iris acts as a light regulator by manipulating the pupil size. The fibrous matrix that makes up the Iris is muscular and can expand, which enlarges the pupil to let more of our light ray into the eye or contract to let less light into the eye. In dim illumination such as nighttime, the Iris will expand so that more light can enter and we can see better. In bright illumination, the Iris contracts, making the pupil smaller so less light can get in and things won't appear too bright.