The color of the human eye is determined primarily by the amount of melanin pigment within the iris, the colored ring surrounding the pupil. More melanin results in darker eye color; brown eyes have the highest concentrations, blue eyes the least. This trait is polygenic, influenced by multiple inherited genes, and is not static. Eye color changes can result from natural developmental processes in infancy, medical conditions, or specific medications in adulthood.
Why Eye Color Changes Naturally in Infancy
The most common reason for a significant eye color change occurs in the first years of life. Many babies, particularly those of Caucasian descent, are born with eyes that appear blue or gray because the specialized cells, called melanocytes, have not yet begun full pigment production. Since the womb is a dark environment, these pigment-producing cells are not fully stimulated until after birth.
Exposure to light triggers the melanocytes in the iris to start producing melanin, a process that unfolds gradually over several months. If the cells produce only a small amount of pigment, the eyes will likely remain blue or transition to gray. A moderate increase in melanin production results in eyes becoming green or hazel.
If the melanocytes are highly active, the eye color will darken to brown. This developmental shift typically begins between three and nine months of age, though subtle changes can continue until a child is three years old. The final, stable eye color is established once melanin production stabilizes in response to continuous light exposure and genetic programming.
Medical Conditions and Medications That Alter Color
In adulthood, a true, noticeable change in iris color is rare and often signals an underlying health issue or a side effect of treatment. One class of glaucoma medications, known as prostaglandin analogs, can cause a permanent darkening of the iris. These eye drops, such as latanoprost and bimatoprost, stimulate melanocytes to synthesize more pigment, causing a lighter blue or green iris to transition to a brown color.
Several pathological conditions can also lead to a color shift, often affecting only one eye, a condition called heterochromia. Fuchs’ heterochromic iridocyclitis is a chronic inflammation of the uvea that causes the affected iris to lose pigment, making it appear lighter than the other eye. Conversely, an inflammatory condition like uveitis, or a severe eye injury, can make the iris appear darker due to swelling, cellular debris, or pigment dispersion.
Pigment dispersion syndrome involves pigment rubbing off the back of the iris and floating within the eye. This loss of pigment can cause a lightening in areas of the iris and may clog the eye’s drainage system, sometimes leading to pigmentary glaucoma. A noticeable, sudden, or painful change in eye color should always prompt an immediate examination by an eye care professional.
Perceived Changes and Subtle Shifts in Adulthood
While dramatic color shifts in adults are typically linked to a medical cause, the perception of eye color changing is common and related to external and physiological factors. Environmental lighting plays a significant role, as the way light reflects off the iris can make the color appear more vibrant or muted. The contrast created by clothing or makeup can also create an illusion, such as a green shirt making hazel eyes seem greener.
Physiological changes in the eye’s mechanics also contribute to perceived shifts. When the pupil dilates in dim light or due to emotional response, the size of the dark center increases, making the surrounding iris appear smaller and darker by contrast. Conversely, when the pupil constricts in bright light, more of the pigmented iris is exposed, potentially making the color seem more intense.
True, slow physiological changes can occur over a lifetime due to the aging process. Over many decades, the iris can sometimes lose a small amount of melanin pigment, which may result in a slight lightening of the color. The natural yellowing of the sclera, or white part of the eye, with age can also alter how the adjacent iris color is perceived by making it appear duller or less bright.