The iris is a muscle that expands and contracts to control pupil size. The pupil enlarges in dimmer lighting and grows smaller in brighter lighting. The pupil also shrinks when you focus on near objects, such as a book you are reading.
When the pupil size changes, the pigments in the iris compress or spread apart, changing the eye color a bit.
Certain emotions can change both the pupil size and the iris color. That's why some people say their eyes change colors when they're angry or loving.
Eye color also can change with age. This happens in 10 to 15 percent of the Caucasian population (people who generally have lighter eye colors).
For instance, my once very brown eyes are now hazel, a combination of brown and green. However, some hazel eyes actually get darker with age.
Note that if your adult eye color changes pretty dramatically, or if one eye changes from brown to green or blue to brown (called heterochromia), it's important to see your eye doctor. Eye color changes can be a warning sign of certain diseases, such as Fuch's heterochromic iridocyclitis, Horner's syndrome or pigmentary glaucoma