Eye color, a trait often considered fixed, is determined by a complex interplay of biology, development, and the physics of light. While an adult’s eye color is generally stable, changes can occur due to natural processes in infancy, temporary shifts in light perception, or underlying health issues that affect the eye’s structure. The common curiosity surrounding eyes that seem to change color is rooted in these various mechanisms. Understanding these causes helps distinguish between harmless visual effects and changes that may require medical attention.
The Biology Behind Eye Color
The color of the iris, the muscular ring surrounding the pupil, is primarily determined by the pigment melanin. Specialized cells called melanocytes produce and deposit this pigment within the layers of the iris. High concentrations of melanin result in brown or black eyes, which are the most common colors globally. Lighter eye colors, such as blue, green, and gray, contain lower concentrations of melanin in the front layer of the iris, known as the stroma. The appearance of these lighter shades is a structural color phenomenon called Rayleigh scattering, not actual pigment. When light enters the stroma, it is scattered by tissue fibers, causing shorter blue wavelengths to reflect back out. Green and hazel eyes result from this blue scattering effect mixing with yellowish pigment present in the stroma.
Developmental Changes in Infancy
The most common reason for a permanent eye color change occurs during a person’s first few years of life. Many infants, particularly those of European descent, are born with eyes that appear blue or gray because the melanocytes in the iris have not yet been fully activated to produce melanin. Melanocytes begin to produce and deposit melanin more actively after birth, triggered by light exposure. This gradual increase in pigment causes the eye color to deepen or change over time, with the most noticeable shifts occurring within the first six to nine months. Subtle changes can continue until a child is about three years old.
Factors That Affect Apparent Eye Color
Many perceived shifts in eye color are not structural changes but temporary optical illusions or physiological responses.
Ambient Light
The amount and type of ambient light significantly influences how the iris color is viewed. Natural sunlight, for example, can make lighter eyes appear brighter and more vibrant compared to the dimmer tones of indoor lighting.
Pupil Size
The size of the pupil, the black opening in the center of the iris, also plays a role in apparent color shifts. When the pupil constricts in bright light, the iris is fully visible and the color appears more concentrated. Conversely, when the pupil dilates, the darker pupil takes up more space, which can make the iris appear darker or cause the color around the edge to seem more intense.
Surrounding Colors
The colors surrounding the eye can alter the perceived hue of the iris. Clothing, makeup, or environmental reflections can enhance or draw out underlying tones like green or gold, changing the eye’s appearance without altering the actual melanin pigment.
Medical Causes of Permanent Eye Color Change
A sudden or noticeable change in eye color in an adult should be evaluated by a medical professional, as it can indicate an underlying health issue or a reaction to medication.
Medications
One structural cause is the use of certain medications, particularly prostaglandin analogs used in the treatment of glaucoma. These drops, such as latanoprost and bimatoprost, can permanently stimulate melanocytes to produce more pigment, leading to a gradual and irreversible darkening of the iris. This effect is most pronounced in individuals with mixed-color eyes, like hazel or green, and can lead to a difference in color between the two eyes if only one is treated.
Trauma and Disease
Trauma to the eye can also cause a permanent change by damaging the iris tissue or its blood vessels. An injury can result in the loss of pigment, making the affected eye appear lighter, or it can cause the deposition of iron or calcium, which may give the iris a discolored appearance. Certain inflammatory conditions, such as Fuch’s heterochromic iridocyclitis, are also known to cause a slow loss of pigmentation in one eye, leading to a lighter color and acquired heterochromia. Pigment dispersion syndrome is another condition where pigment granules flake off the back of the iris and can clog the eye’s drainage system, sometimes causing a visible lightening of the iris where the pigment is lost.