The color of the eye's iris can vary dramatically between individuals due to genetic differences. Yet eye color can also vary over time in an individual. Eye color change can happen as a result of natural changes in the body or as a reaction to medications. Sometimes a medical disorder is the cause of a lighter or darker iris pigmentation.
The ring of color in the eye is called the iris. The iris contracts and expands around the central pupil to let in more or less light as needed. When this movement occurs, the color of the iris may appear to change color. Contraction of the iris causes the eye color to appear darker, while expansion of the iris causes the eye color to appear lighter. Strong emotions such as anger or love, which cause the heart rate to rise, may also cause the pupil to contract and change the eye's color.
The eye color of fair-skinned infants is often light blue or slate grey. Melanin is the biological chemical that colors the skin, hair and eyes, causing this light eye color. As the child grows and is exposed to sunlight, the body produces more melanin in response. Increased melanin production in the body causes the infant’s eyes to change color during the first year.
Adult individuals with light eye colors may experience gradual eye color change as they get older. Ten to15 percent of the Caucasian population experiences this gradual change. The color of the eye may get lighter or darker because of changes in melanin production due to advanced age.
Some medications, including glaucoma treatments and medications for growing eyelashes, may cause the iris to change color. Usually these medications are in eye-drop form and cause the iris to permanently darken. The skin around the eye and eye lashes may also darken in color. Hazel eyes are most often affected by these medications.
When the eye color of an adult changes from blue to brown or brown to green dramatically, it is called heterochromia. This type of change could result from a disease such as Fuch’s heterchromic iridocyclitie, Horner’s syndrome or certain types of glaucoma. It is important to see a doctor if this type of change occurs to be correctly diagnosed and treated to prevent vision damage.