People often wonder if eye color can change. The answer is yes, though true and lasting changes in adulthood are uncommon and typically related to health or medication. The color of the iris is determined by the amount of pigment it contains. While this is fixed for most of one’s life, measurable shifts occur during specific periods and circumstances, ranging from natural development in infancy to permanent darkening caused by medical treatments. Perceived changes due to lighting are also common, but these are optical illusions rather than physical transformations of the eye’s structure.
The Biological Basis of Eye Color
The color seen in the eye is primarily determined by the concentration and distribution of melanin, the same pigment that colors skin and hair, within the stroma of the iris. People with brown eyes have a high concentration of melanin, which absorbs most of the light entering the eye. Conversely, those with blue eyes have very little melanin in the front layer of the iris.
The appearance of lighter colors like blue, green, and hazel is actually an optical effect, not the result of blue or green pigment being present. When melanin levels are low, light entering the stroma scatters and reflects back, a phenomenon similar to Rayleigh scattering that makes the sky appear blue. Green and hazel eyes result from a moderate amount of melanin mixing with this light scattering effect. Eye color is a complex, polygenic trait, meaning multiple genes, notably OCA2 and HERC2, work together to determine the final shade.
Developmental Changes in Infancy
The most common instance of eye color change occurs in babies shortly after birth. Many infants are born with eyes that appear blue, gray, or slate-colored because the melanocytes in their irises have not yet begun full melanin production. These pigment-producing cells are not fully activated until they are exposed to light outside the womb.
Over the first six to twelve months of life, and sometimes continuing until age three, the melanocytes gradually increase their melanin output. This accumulation of pigment causes the eye color to darken, often shifting from blue or gray to green, hazel, or brown. If the melanocytes produce a high concentration of melanin, the eyes will become brown, remaining stable for the rest of their life. This developmental change is a natural process.
Changes Triggered by Health and Environment
Permanent changes in adult eye color can be a physical manifestation of specific medical conditions, eye trauma, or medication side effects. A sudden or noticeable change in color should always be evaluated by an ophthalmologist, as it can be an indicator of an underlying health problem. Conditions like Fuch’s heterochromic iridocyclitis, a rare form of chronic inflammation, can cause a loss of iris pigment, leading to a lightening of the affected eye.
Other disorders, such as pigment dispersion syndrome, involve the loss of pigment from the back surface of the iris, which can then clog the eye’s drainage system and lead to glaucoma. Eye trauma, such as a blunt injury, can also disrupt the tissue of the iris, causing a visible shift in color. Heterochromia, where an individual has two different colored eyes or two different colors within one eye, can be present at birth or develop later due to these diseases or injuries.
Certain prescription eye drops, specifically prostaglandin analogs used to treat glaucoma, can permanently increase iris pigmentation. Medications like latanoprost or bimatoprost can cause a gradual darkening of the eye color, particularly in individuals with lighter irises like green or hazel. This side effect occurs because the medication stimulates melanocytes to produce more pigment, and the change is often irreversible.
Illusory and Temporary Shifts
Many perceived changes in eye color are not physical alterations to the iris but are instead temporary optical phenomena. The size of the pupil, the black opening in the center of the iris, constantly changes in response to light levels and emotional state. When the pupil dilates in dim light or due to excitement, the dark center expands, making the surrounding iris appear slightly darker or more intense.
Conversely, when the pupil constricts in bright light, the iris fills more of the visual field, which can make the color appear lighter. The perceived color is also heavily influenced by external factors, such as the color of clothing, makeup, and ambient light. These shifts in appearance are entirely cosmetic and do not involve any physical change to the eye’s structure.