Eye color, defined by the pigmentation of the iris, is generally established early in life, but it can change with age. While the most dramatic shifts occur during infancy, adult eyes can experience subtle, natural alterations over decades. Significant or rapid changes in an adult’s eye color are uncommon, and when they do happen, they often warrant consultation with an eye care professional.
The Role of Melanin in Eye Color
The color of the iris is fundamentally determined by the amount and distribution of a pigment called melanin. Melanin is produced by specialized cells known as melanocytes, which are located within the iris. The iris has two main layers containing this pigment: the stroma (front layer) and the iris pigment epithelium (back layer).
The concentration of melanin in the stroma dictates the resulting eye color. Eyes with a high concentration of melanin absorb most of the light entering the eye, resulting in darker shades like brown or black. Brown is the most common eye color globally because of this high pigment level.
Eyes that appear blue, green, or gray have a much lower concentration of melanin in the stroma. These colors are not caused by blue or green pigment, but rather by a physical phenomenon called Rayleigh scattering. With less pigment, shorter, blue wavelengths of light are scattered by the collagen fibers in the stroma, making the eye appear blue, similar to the sky. Green and hazel eyes represent intermediate amounts of melanin, which combine with the light scattering effect to produce their unique hues.
Why Infant Eyes Change Color
The most significant period for eye color change occurs during a person’s first few years of life. Many babies, particularly those of European descent, are born with eyes that appear blue or gray. This lighter initial color is because their melanocytes have not yet been fully activated to produce their final, genetically determined amount of melanin.
During gestation, the eyes are shielded from light, and the system responsible for producing iris pigment is not fully active. Once exposed to natural light after birth, the melanocytes begin to stimulate melanin production. This gradual increase in pigment causes the eyes to darken or change color over time, a process that typically begins around six months of age.
The final eye color often stabilizes between six months and one year, but this developmental change can continue for up to three years. If a child is genetically predisposed to brown eyes, continued melanin production will cause the initial blue-gray color to shift to a darker shade. Babies who retain a light eye color produced only a small amount of melanin, resulting in a stable blue or green hue.
Subtle Shifts and Color Changes in Adulthood
Once eye color has stabilized after childhood, it tends to remain consistent for the rest of a person’s life, but subtle, natural shifts can still occur. Over many decades, some individuals may notice a minor lightening or darkening of their eye color. This slow change is thought to be related to gradual, age-related fluctuations in melanin concentration within the iris.
More often than a true biological shift, what people observe are perceived changes in eye color. Factors like ambient lighting, the colors of clothing, and makeup can dramatically alter how the iris hue is interpreted. Pupil dilation, which changes the ratio of the colored iris to the dark pupil, can also make eyes appear to change color, especially in response to strong emotions or low light.
Sudden or dramatic color shifts in a mature adult are rare and can indicate an underlying medical condition. Certain diseases, such as Fuchs heterochromic iridocyclitis, can cause a loss of iris pigmentation, leading to a noticeable color change. Pigment dispersion syndrome is another condition where pigment granules flake off the back of the iris, which can lighten the eye color and potentially lead to glaucoma. Other causes include eye injury, inflammation, or the long-term use of specific medications, such as some prostaglandin analogs used to treat glaucoma, which can cause the iris to darken.