A person’s eye color is often considered a fixed trait, but the reality is that the color of the iris can undergo significant changes over a lifetime. These shifts in coloration are not limited to childhood development; they can also occur in maturity due to specific external factors, underlying health conditions, or the natural process of senescence. Understanding the true nature of eye color requires moving past the idea of a stable, unchangeable adult hue, as changes can manifest in distinct and medically relevant ways long after the developmental period ends. The color you see in the mirror is a dynamic biological feature that can respond to various internal and external stimuli throughout the years.
The Mechanism of Eye Color
The color of the eye is a polygenic trait determined by two main factors: the amount of the pigment melanin present and the way light interacts with the structure of the iris. Melanin is produced by specialized cells called melanocytes, and its concentration in the iris stroma, the front layer, is the primary determinant of color. Eyes with a high concentration of melanin absorb most incoming light, resulting in brown or black coloration.
For blue, green, and hazel eyes, the physical structure of the iris plays a dominant role because these colors do not come from a specific pigment. Instead, they result from an optical phenomenon known as Rayleigh or Tyndall scattering. When light enters the stroma, the low melanin content allows the shorter, blue wavelengths of light to scatter back out, while the longer wavelengths are absorbed by the dark pigment epithelium beneath. Green eyes are a combination of this light scattering with small amounts of a yellowish pigment, creating the perceived composite hue.
Eye Color Stabilization During Childhood
The most dramatic and common change in eye color occurs during the first few years of life, which is a normal part of human development. Many infants are born with eyes that appear blue or gray because the melanocytes in the iris have not yet been fully activated to produce their predetermined amount of melanin. Melanin production is a gradual process that is often stimulated by exposure to light after birth.
As the melanocytes begin to synthesize and deposit more pigment into the stroma, the initial light color deepens or changes completely. For most individuals, the final, stable adult eye color is established sometime between six months and three years of age, though some sources suggest it can take up to six years. While the majority of people have a fixed color by early childhood, a small subpopulation, approximately 10% to 15% of white subjects in one study, may continue to experience subtle shifts throughout adolescence and into adulthood.
Causes of Color Change in Adulthood
When eye color changes occur after the developmental period, they are often linked to specific external factors or underlying medical conditions that alter the iris’s pigment or structure.
One common cause of adult eye color change is the long-term use of certain prescription medications, particularly prostaglandin analogs used to treat glaucoma. These drops, such as latanoprost or bimatoprost, can permanently increase the melanin content in the iris, typically causing a permanent darkening of the eye color. This darkening is often more noticeable if the medication is only used in one eye.
Diseases that affect the eye’s internal structures can also lead to noticeable color changes, frequently resulting in two differently colored eyes, a condition known as heterochromia. Fuchs heterochromic iridocyclitis is a rare inflammatory condition that can cause a gradual loss of pigment from the iris, making the affected eye appear lighter or faded over time.
Conversely, pigmentary glaucoma involves the shedding of pigment granules from the back surface of the iris, which can then clog the eye’s drainage angle, and may cause the iris to appear lighter where the pigment has fallen away. Physical trauma to the eye can also cause a sudden or lasting color change by disrupting the delicate tissue of the iris. An injury can lead to a loss of pigment or, in some cases, the deposition of blood products or scarring that alters the eye’s appearance.
Another neurological condition, Horner’s syndrome, can sometimes cause the affected eye to have a lighter color due to a disruption in the nerve pathway that controls melanin production. Any noticeable, non-developmental shift in eye color warrants examination by an eye care professional to rule out a medical issue.
Subtle Color Shifts Related to Normal Aging
Beyond pathological causes, the eyes can experience subtle, non-disease-related changes simply as a result of the aging process over decades. The pigment in the iris, like the pigment in hair and skin, can undergo gradual loss or atrophy. This slow pigment reduction can cause the iris to appear slightly duller or lighter in color than it was in young adulthood.
A common age-related change that affects the perception of eye color is the development of Arcus Senilis, which is not a change in the iris itself. This condition presents as a white, gray, or bluish ring that forms on the outer edge of the cornea, the clear dome covering the iris. The ring is caused by lipid deposits, typically cholesterol, and is a normal part of aging, especially in people over 60. Although Arcus Senilis does not change the actual color of the iris, the presence of the white ring can create the visual illusion that the eye’s color has faded or has two distinct zones of coloration.