The iris, the colored part of the eye, is a complex muscular diaphragm that controls the size of the pupil. While eye color is genetically determined and fixed for most of adult life, the iris can change color under specific conditions. The potential for change depends on when it occurs and what causes it.
The Science Behind Eye Color
The color of the iris is determined by the concentration of melanin within the iris stroma, the front layer of the iris. Higher concentrations of melanin lead to darker eyes, such as brown, the most common eye color globally. Lower melanin levels result in lighter eye colors like blue, green, or hazel.
In eyes with little melanin, the appearance of blue is not due to a blue pigment but is a structural color effect. Light entering the stroma is scattered back, and because shorter blue wavelengths scatter more easily, the eye appears blue in a phenomenon similar to the Tyndall effect. Green and hazel eyes result from a combination of this light scattering and a moderate presence of yellowish-brown melanin pigment. Eye color is a polygenic trait, influenced by multiple genes, including OCA2 and HERC2, which regulate melanin production and distribution.
Developmental Changes in Eye Color
The most common instance of iris color change occurs during infancy. Many babies, particularly those of Caucasian descent, are born with eyes that appear blue or gray because the specialized cells that produce melanin, called melanocytes, are not fully activated at birth.
Exposure to light stimulates these melanocytes to begin producing and depositing melanin pigment into the iris stroma. As melanin increases, the eye color gradually darkens and shifts to its permanent shade. This process typically begins between six months and one year of age, but the final color may not be established until a child is between three and six years old.
Causes of Acquired Color Change in Adults
True, permanent color change in one or both adult eyes is rare and often signals an underlying medical issue. This change, known as acquired heterochromia, involves the darkening or lightening of the iris pigment. One cause is Fuch’s heterochromic iridocyclitis, a chronic, low-grade inflammation of the iris that often causes the affected eye to gradually lose pigment and lighten.
Certain medications can also induce a permanent color change, notably prostaglandin analogs like latanoprost, used to treat glaucoma. These drops can stimulate melanocytes to produce more pigment, leading to a permanent darkening of the treated eye, particularly in individuals with mixed-color irises. Less commonly, pigmentary glaucoma can cause subtle color changes when pigment flakes off the back of the iris due to friction. Any noticeable, non-temporary change in adult eye color warrants an immediate evaluation by an ophthalmologist.
Factors That Make Eye Color Appear Different
While true pigment change is uncommon in adults, the apparent color of the eyes can shift dramatically based on external factors. The most significant factor is the quality and intensity of ambient lighting. Under direct sunlight, lighter eye colors appear more vibrant because the light scattering effect is maximized, while in dim light, the color may seem duller or darker.
The size of the pupil also influences perceived color. When the pupil constricts in bright light, the darker central portion shrinks, and the full expanse of the colored iris is visible, potentially making the color seem more intense. Conversely, when the pupil dilates in low light, the large black center can make the surrounding iris color appear darker or less prominent. Surrounding colors, such as those from clothing or makeup, can also create optical contrast that makes the eye color appear to shift without any actual change to the iris pigment.