The idea that a person’s eye color is fixed for life after early childhood is a common assumption, but it is not entirely accurate. While a stable adult eye color is the norm, the shade of the iris can shift due to several factors throughout a person’s lifespan. These changes range from natural developmental processes in infancy to subtle perceived shifts influenced by the environment, and even permanent, medically induced alterations. Understanding the biology of eye color reveals why these shifts occur.
How Eye Color is Determined
The color seen in the iris is determined by two main factors: the concentration of the pigment melanin and the physics of light interaction. Melanin is produced by cells called melanocytes. The amount of this pigment present in the front layer of the iris determines the color. High concentrations of melanin result in brown eyes, the most common eye color globally, while lower levels lead to lighter shades.
Lighter eye colors, such as blue and green, are not caused by blue or green pigment but by a phenomenon called Rayleigh scattering. When light enters an iris with very little melanin, the shorter, blue wavelengths are scattered back out of the dense tissue, making the eyes appear blue, similar to how the sky looks blue. Green and hazel eyes result from a moderate amount of yellowish-brown melanin combined with this blue light scattering effect, creating an overall green or mixed appearance.
The Natural Color Shift in Infants
The most common instance of eye color change occurs during infancy, as the eyes adjust to life outside the womb. Most babies are born with eyes that appear blue or grayish because the pigment-producing melanocytes in the iris have not yet been fully activated. Melanin production is minimal at birth.
Upon exposure to light, the melanocytes begin to increase melanin production. If a child is genetically programmed to produce a large amount of melanin, their eyes will darken from blue or gray to hazel or brown. The most significant changes happen between six and twelve months of age, becoming more noticeable as the iris accumulates pigment.
The eye color generally stabilizes by a child’s first birthday, though subtle shifts can continue until they are three years old. This period of developmental pigmentation is the only time an eye color change is considered a normal part of maturation. After this point, any permanent change in color is rare and may indicate an underlying cause.
Causes of Permanent Eye Color Change in Adults
A permanent change in adult eye color, known as acquired heterochromia when only one eye is affected, often signals a medical issue or the effect of medication. Certain pharmaceutical agents, particularly prostaglandin analogs used to treat glaucoma, cause a permanent darkening of the iris. These eye drops stimulate melanogenesis, which leads to a gradual shift, often turning lighter colors toward brown. This darkening is irreversible, even after the medication is stopped.
Several medical conditions can also lead to a change in iris pigmentation or appearance. Fuchs’ heterochromic iridocyclitis, a form of chronic, low-grade inflammation, can cause the affected iris to lighten due to pigment loss. Pigment dispersion syndrome is another condition where pigment granules flake off the back of the iris and float within the eye’s fluid. This loss of pigment can cause the iris to lighten in patches. The dislodged pigment can sometimes clog the eye’s drainage system, leading to a form of glaucoma.
In rare cases, conditions affecting the nervous system, such as Horner’s syndrome, can impact the muscles that control the iris and pupil, leading to one pupil being smaller and the affected eye appearing lighter. Trauma to the eye can also cause localized damage to the iris tissue, resulting in a visible change in color or pattern. Any sudden alteration in adult eye color warrants an examination by an eye care professional.
Temporary and Perceived Eye Color Changes
Many people report that their eye color seems to change from day to day, which is usually a temporary or purely visual effect. The most significant factor influencing this perception is ambient lighting. Different light sources reflect off the melanin and stroma in the iris, making the color appear to shift between shades of green, blue, or gray.
Changes in pupil size also contribute to the perception of a color change because the iris is a muscular diaphragm around the pupil. When the pupil constricts in bright light, the color appears more concentrated. Dilation in dim light exposes more of the iris’s darker, peripheral edge. This change in the ratio of the black pupil to the colored iris can make the eye appear to be a different shade.
Emotional states can also cause a perceived shift, as strong feelings like excitement or fear trigger the body’s fight-or-flight response, which causes the pupils to dilate. The color of clothing or makeup can create a visual illusion by reflecting hues onto the eye. These perceived changes are entirely optical and do not involve any alteration to the actual pigment within the iris.