The color of the eye is primarily determined by the concentration of the pigment melanin within the iris stroma. Greater amounts of melanin result in darker colors like brown, while lower concentrations lead to lighter colors such as blue or green. While genetics largely predetermines the color, the final shade is the result of both the pigment’s concentration and the scattering of light. For a healthy adult, eye color is generally considered stable, and permanent lightening in adulthood is rare, often linked to specific conditions.
Why Infant Eye Color Changes
The most common instance of eye color changing occurs during a baby’s first year of life. Most infants are born with blue or grayish eyes because the specialized cells that produce melanin, called melanocytes, have not been fully activated yet. Without sufficient melanin, light scatters within the iris tissue, creating the blue-gray appearance. Exposure to natural light stimulates melanocytes to begin producing and depositing melanin into the iris, causing the color to darken or “set.” Significant changes typically occur between three and nine months, but the final color may not be reached until the child is one year old, or up to three years of age.
Factors That Make Eyes Appear Lighter
Many perceived changes in adult eye color are not due to pigment alteration but are shifts in appearance or perception. The size of the pupil plays a role in how the surrounding iris color is viewed. When the pupil dilates due to low light or emotional response, it exposes less of the iris, making the remaining colored portion appear lighter by contrast. Conversely, when the pupil constricts in bright light, it covers more of the iris, sometimes making the color look darker. External factors like clothing color or ambient lighting, which influences how light is scattered and reflected, can also create the illusion of a color change.
Permanent Changes in Adult Eyes
While eye color is mostly fixed in adulthood, true permanent changes can occur due to aging, medication, or underlying medical conditions. With age, some individuals may experience a slight, gradual lightening of their eye color due to a natural loss of melanin pigment from the iris stroma. This benign fading is considered a normal part of the aging process.
A more significant, often asymmetrical, lightening can signal a medical issue. Examples include Fuchs’ heterochromic iridocyclitis, a rare inflammatory disorder causing pigment loss in the affected eye. Pigment dispersion syndrome is another condition where pigment granules flake off the back of the iris, leading to localized lightening. This flaked pigment can clog the eye’s drainage system and increase the risk of glaucoma.
Certain glaucoma medications, specifically prostaglandin analogs like latanoprost, can cause a permanent change in eye color, usually resulting in a darkening of the iris. However, the development of an eye that is suddenly, noticeably lighter, especially in only one eye, requires evaluation by an ophthalmologist. Such a distinct shift can indicate an underlying condition, iris atrophy, or the long-term effects of trauma.