Eye color is often considered a fixed, lifelong trait determined by genetics. While this is largely true for most adults, the color of the iris is not entirely static. The perceived and actual shade of the eye can change due to physical effects, developmental milestones, specific medications, and underlying medical conditions. Shifts occur through changes in pigment concentration or how light interacts with the eye’s structure.
The Biological Mechanism of Eye Color
The color seen in the eye is determined by the amount of the pigment melanin present in the iris’s front layer, known as the stroma. Specialized cells called melanocytes produce this melanin, which is the same pigment responsible for skin and hair color. A high concentration of melanin results in darker eye colors, such as brown or black, because the pigment absorbs most of the incoming light.
Lighter eye colors like blue, green, and gray occur when the stroma contains low to moderate amounts of melanin. Since there is not enough pigment to absorb all the light, the light enters the stroma and scatters back out. This scattering phenomenon, similar to Rayleigh scattering, makes the eyes look blue. Green and hazel eyes result from a combination of this light scattering with small amounts of yellowish-brown melanin.
Natural and Perceived Changes Over a Lifetime
The most common instance of eye color change occurs naturally during infancy. Most babies of European descent are born with blue or gray eyes because the melanocytes in their irises have not yet begun full melanin production. Exposure to light after birth stimulates these cells, and the eyes begin to accumulate pigment. The final eye color typically stabilizes between six and twelve months of age, with more subtle shifts sometimes continuing until a child is around three years old. Later in life, minor changes can happen due to natural aging processes, which may involve a slight darkening or lightening of the iris pigment.
Many instances where people believe their eye color has changed are actually perceived shifts caused by external factors. The color of clothing, surrounding light, and makeup can all affect how the iris appears. Pupil dilation is another factor that creates a perceived change in color. When the pupil enlarges, the dark background of the eye’s interior becomes more prominent, making the iris appear darker or more intense. This optical effect does not represent any change to the actual pigment within the iris tissue.
Pharmaceutical Causes of Permanent Color Shifts
Certain types of eye drop medications can cause a genuine, permanent change in iris pigmentation. The most well-known examples are prostaglandin analogs, such as latanoprost and bimatoprost, which are commonly prescribed to treat glaucoma. The mechanism involves the drug stimulating melanogenesis, the process of melanin production, within the iris melanocytes. This stimulation typically causes lighter-colored irises—specifically blue, green, or hazel eyes—to darken into a brown shade. The increased pigmentation is often progressive and tends to be permanent, even after the medication is discontinued. This side effect appears to occur more frequently in patients with mixed-color irides.
Diseases and Conditions That Alter Pigmentation
Pathological changes to eye color are often medically significant and frequently affect only one eye, resulting in acquired heterochromia—a difference in color between the two irises. One condition that causes this is Fuchs’ heterochromic iridocyclitis (FHI), a chronic, low-grade inflammation affecting the uvea, the middle layer of the eye. FHI leads to depigmentation and atrophy of the iris in the affected eye, causing it to become lighter or have a faded appearance compared to the unaffected eye.
Trauma to the eye can also trigger changes in pigmentation. An injury may cause pigment to be deposited onto the lens or cornea, or it might lead to pigment loss from the iris tissue. Another condition, Pigment Dispersion Syndrome, involves pigment granules flaking off the back of the iris and distributing throughout the eye, which can lead to a form of glaucoma.
Neurological disorders can also affect eye color through their impact on the sympathetic nervous system. Horner’s Syndrome results from damage to the nerve pathway connecting the brain to the face and eye. If this nerve damage occurs before the age of two, it can cause iris heterochromia because sympathetic nerves regulate iris pigmentation development. This damage prevents the affected iris from developing its full color, leaving it permanently lighter than the other eye. Any sudden, unexplained, or unilateral change in eye color should be evaluated immediately by an ophthalmologist, as it can signal a serious underlying medical issue.