Eye color is generally stable, but apparent shifts can spark curiosity. While typically set early in life, various factors can make eye color appear different, and in rare instances, a true biological change can occur. Understanding these influences clarifies why brown eyes might sometimes seem to take on a greenish tint.
The Science of Eye Color
Eye color primarily depends on the amount and type of melanin in the iris, the colored part of the eye. Eumelanin (black-brown) and pheomelanin (yellow-red) are the two main types. Brown eyes have a high concentration of melanin, particularly eumelanin, which absorbs most light.
Green eyes result from a lower melanin concentration combined with pheomelanin and Rayleigh scattering. This process, which makes the sky appear blue, involves the scattering of shorter, blue wavelengths of light by collagen fibers in the iris. When this blue scattering combines with the yellowish tint from pheomelanin, a green appearance emerges.
Factors Influencing Perceived Eye Color
Many instances of “changing” eye color are optical illusions or environmental effects rather than biological alterations. Lighting plays a significant role, as different light sources (e.g., natural sunlight versus indoor artificial light) can emphasize various undertones in the iris. The amount of light can also cause the pupil to expand or contract, subtly changing its perceived color.
Surrounding colors, such as those from clothing, makeup, or the environment, can reflect onto the eye and influence how its color is perceived. For instance, wearing green clothing might enhance any existing green flecks in a hazel or light brown eye, making the entire iris appear greener. Emotional states can also affect pupil size, momentarily altering the visible iris area and creating a temporary perception of color change. These factors do not alter the actual pigment in the eye but manipulate how light interacts with it.
Natural Biological Shifts in Eye Color
While eye color is largely genetic, some natural biological shifts occur, particularly in early life. Many babies are born with lighter eyes, often blue or gray, because their melanin-producing cells haven’t fully developed or been exposed to light. As infants grow (typically 3 months to 3 years), increased light stimulates melanin production, often darkening eye color. This explains why some babies born with blue eyes might later develop brown or hazel eyes.
Significant natural changes from brown to green are rare in adults. Minor shifts can occur with age, such as slight lightening due to pigment degradation, but dramatic transformations are not typical without an underlying reason. Some individuals may experience subtle eye color changes throughout adolescence and adulthood, though these are generally within a limited range.
Medical Causes of Eye Color Change
True, lasting changes in eye color in adults often indicate an underlying medical condition or external factor. One such condition is heterochromia, where an individual has two different colored irises or multiple colors within a single iris. This can be present from birth or acquired later.
Fuchs heterochromic iridocyclitis (FHI) is an inflammatory condition that can cause the affected eye to lighten due to pigment loss from the iris, potentially making a brown eye appear greener or lighter. Horner’s syndrome, a neurological disorder, can also result in the affected eye becoming lighter, particularly if it develops in childhood. Pigment dispersion syndrome occurs when pigment from the back of the iris flakes off and can clog the eye’s drainage system, sometimes leading to pigmentary glaucoma. This pigment loss can alter the iris’s appearance, potentially lightening it. Inflammation of the iris, known as iritis or anterior uveitis, can also lead to changes in eye color or appearance. Certain medications, such as some prostaglandin analog eye drops used for glaucoma, can cause permanent darkening of eye color, usually in lighter irises, though some reports suggest they could cause shifts.
When to Seek Professional Advice
While many perceived eye color changes are harmless optical phenomena, any sudden, noticeable, or sustained alteration in eye color in an adult warrants medical evaluation. It is important to consult an eye care professional if the color change is accompanied by other symptoms. These symptoms include eye pain, redness, blurred vision, light sensitivity, or changes in pupil size or shape. An ophthalmologist or optometrist can determine if the change is a benign variation or a sign of an underlying health issue requiring attention.