The iris is the visible, colored part of the eye, functioning by regulating the pupil’s size to control the amount of light reaching the retina. The color is determined by melanin, the same pigment that colors skin and hair. While eye color is typically stable throughout adulthood, changes can occur under specific and sometimes medically significant circumstances.
The Foundation of Eye Color
The concentration of melanin within the stroma, the connective tissue layer of the iris, dictates the final eye color. Brown eyes possess a high concentration of melanin, which efficiently absorbs most light wavelengths entering the eye. This dense pigmentation results in the deep, rich brown hue.
In contrast, blue eyes contain very little melanin in the stroma, and the color is not caused by pigment. Instead, the color is a structural effect resulting from the scattering of light known as Rayleigh scattering. Shorter, blue wavelengths of light are scattered back out of the eye by the stroma’s fibers, similar to how the sky appears blue. Green and hazel eyes represent intermediate levels of melanin, creating a blend of light absorption and scattering effects.
Natural Changes in Early Life
The most common shift in iris color occurs during infancy, as the pigment-producing cells, or melanocytes, begin to activate. Many babies, particularly those with fair complexions, are born with eyes that appear blue or gray because the full production of melanin has not yet commenced. Light exposure after birth triggers the melanocytes to increase their output, leading to the gradual darkening of the iris.
The most noticeable color changes typically take place between three and twelve months of age, when a baby’s eyes often settle into their permanent shade. While most children have their stable adult eye color by their first birthday, subtle shifts can sometimes continue until around three years of age. Once a light color darkens to brown, it is highly likely to remain permanent, as the underlying pigmentation mechanism has stabilized.
Changes Caused by External Factors
Permanent changes in adult eye color are rare and often point to an acquired medical condition or a reaction to specific treatments. One notable cause is the use of certain prostaglandin analog eye drops prescribed to manage glaucoma. These medications can cause the iris to permanently darken by stimulating melanogenesis, the process of melanin production. This darkening effect is more frequently observed in individuals who already have mixed-color irides, such as green or hazel eyes.
Certain diseases can also lead to a change in eye color. When only one eye is affected, this phenomenon is called heterochromia. Conditions like Fuchs’ heterochromic iridocyclitis cause low-grade inflammation that results in the affected iris losing pigment and appearing lighter. Acquired Horner’s syndrome, a disorder of the nervous system, can similarly lead to a decrease in pigmentation in the affected eye.
Physical trauma to the eye can also induce a permanent alteration in color appearance. A severe blow can damage the iris tissue, leading to localized tissue loss or the dispersion of melanin pigment cells. This damage can create transillumination defects where light passes through areas of the iris that are now thinner, altering how the eye’s color is perceived. Bleeding inside the eye, known as hyphema, can temporarily stain the iris, leading to a discoloration.
Misconceptions About Color Shifts
Many perceived color changes are simply visual effects rather than genuine biological alterations to the iris pigment. The quality and type of ambient lighting significantly impact how an eye’s color is interpreted, as structural colors like blue and green are highly dependent on the way light scatters. For instance, certain light sources can emphasize the flecks of gold or green in a hazel eye, making the entire iris appear to have shifted hue.
The size of the pupil also plays a role in the perceived color of the iris. When a person is excited, fearful, or in dim light, the pupil dilates, revealing the darker, less pigmented outer ring of the iris. Conversely, a constricted pupil in bright light highlights the central, denser pigment, which can temporarily make the eye appear a different shade, yet the actual melanin content remains unchanged.