The colored part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera.  The round opening in the center of the iris is called the pupil.  The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.  

The sphincter muscle lies around the very edge of the pupil.  In bright light, the sphincter contracts, causing the pupil to constrict.  The dilator muscle runs radially through the iris, like spokes on a wheel.  This muscle dilates the eye in dim lighting.  

The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber).  Its color comes from microscopic pigment cells called melanin.  The color, texture, and patterns of each person's iris are as unique as a fingerprint.