Blue eyes, often considered stable after childhood, can change color, though this is not a frequent occurrence in adults. While the genetics determining eye color are fixed, the iris’s appearance can be influenced by infant development, specific medical conditions, medications, or visual perception.
The Mechanism Behind Blue Eye Color
The blue color in an iris is not caused by blue pigment, but is a structural color effect. The iris has two primary layers: the epithelium at the back, which contains dark pigment, and the stroma at the front, which is mostly colorless collagen fibers. Blue eyes have a very low concentration of melanin, the brown pigment responsible for darker eye colors, within the stroma.
When light enters the eye, it passes through the stroma and interacts with the tissue’s fibers. This interaction causes shorter, blue wavelengths of light to scatter back out, a physical phenomenon known as Rayleigh scattering. Longer wavelengths are absorbed by the dark epithelium. The resulting blue color is an optical illusion dependent on available lighting conditions.
Natural Color Shifts During Early Childhood
The most common time for blue eyes to change color is during the first few years of life. Many babies are born with eyes that appear blue or grayish because their melanocytes, the pigment-producing cells in the iris, have not yet been fully activated. Melanin production is low in the womb due to the lack of light exposure necessary to stimulate these cells.
Once exposed to light after birth, melanocytes begin to produce and deposit more melanin into the iris stroma. If production increases, the color can shift permanently. This developmental change typically occurs between six months and three years of age, though subtle changes can continue longer. An increase in melanin concentration can cause the eyes to deepen in color, potentially transitioning from blue to green, hazel, or brown.
Factors That Cause Color Change in Adulthood
Pathological and Medical Changes
A noticeable, permanent change in adult eye color is rare and often signals an underlying medical concern. Certain diseases can cause the iris to lose pigment, such as Fuchs heterochromic iridocyclitis, an inflammatory condition that results in a lighter, different-colored iris in one eye. Conversely, conditions like pigment dispersion syndrome can cause pigment granules to flake off the back of the iris and redistribute, potentially making parts of the eye appear darker or lighter.
Specific medications are also known to cause permanent color shifts, particularly prostaglandin analogues, a class of glaucoma eye drops. These drops, used to lower eye pressure, stimulate melanocytes to produce more pigment. This results in the iris darkening over time, often turning a blue or green eye to a brownish hue. Since these changes are often asymmetrical, affecting only the treated eye, they can result in acquired heterochromia, a condition where the eyes are two different colors. Any sudden or lasting change in adult eye color should be evaluated by an ophthalmologist.
Perceived and Temporary Changes
While biological changes are rare, the appearance of blue eyes changing color is quite common, often due to external factors or visual illusions. Because blue eye color is a structural effect based on light scattering, it is highly sensitive to the color and intensity of ambient light. Different lighting—such as sunlight versus indoor light—alters how blue wavelengths are scattered and perceived, making the eyes seem to shift in tone.
The size of the pupil also influences perceived color by changing the visible iris ratio. When the pupil dilates in dim light or due to emotional response, the dark inner edge expands, which can make the remaining blue area look more intense or different. Furthermore, the colors worn near the face, such as clothing or makeup, can reflect onto the iris, temporarily altering its hue through a contrast effect. These temporary shifts are purely optical and do not represent any physical change to the melanin content of the iris.