A shift in perceived eye color is common in infants but rare in adults. A significant, permanent change in an adult’s eye color often signals an underlying biological or medical factor. Most perceived changes are temporary and related to the physics of light, but any lasting alteration warrants professional attention from an eye care specialist.
How Eye Color is Determined
Eye color is determined by multiple genes, but the final hue depends largely on the amount of melanin present in the iris. The iris, the colored part of the eye, consists of a back layer and a front layer called the stroma. The color we see is determined by the melanin concentration in the stroma.
Brown eyes contain high concentrations of melanin in the stroma, which absorbs most light. Blue eyes, in contrast, have very little or no melanin pigment. The absence of this pigment causes the stroma’s collagen fibers to scatter light that enters the eye.
This light scattering is known as Rayleigh scattering, the same effect that makes the sky appear blue. When light hits the low-pigment stroma, shorter blue wavelengths scatter back out, creating the perception of blue. Green and hazel eyes result from a moderate amount of melanin combined with this light scattering effect.
Infants often have lighter eyes, sometimes appearing blue, because the specialized melanin-producing cells (melanocytes) have not yet fully activated. As the baby is exposed to light, melanin production increases. The final, permanent eye color typically develops between nine months and three years of age. After this period, the natural pigment of the iris is generally set for life.
Temporary and Perceived Color Shifts
Many perceived changes in eye color are not biological shifts in pigment but illusions created by the surrounding environment. The quality of light is a primary factor in perception. Different light sources, such as warm indoor lighting versus cool natural daylight, affect how light scatters within the iris, making a light-colored eye appear more vivid or dull.
The size of the pupil also influences the apparent color of the iris. The pupil, the black center of the eye, changes size in response to light and emotional state. When the pupil constricts in bright light, the full spectrum of the iris is exposed, which makes the color appear more intense or vibrant.
When the pupil dilates in low light or due to strong emotions, the exposed iris shrinks. The dominant black of the pupil can make the eye color appear darker or less distinct. For people with lighter eyes, this movement can temporarily compress the pigment, making the color appear concentrated or enhanced.
External factors reflecting onto the eye also create this illusion. Wearing clothing or makeup in shades of blue or green can reflect those hues onto the iris, intensifying the perception of existing blue or green tones. This contrast emphasizes the underlying color, making the eye appear to have shifted when only the surrounding perception has changed.
Permanent Medical Causes of Eye Color Change
A permanent change in adult eye color warrants evaluation by an ophthalmologist. One such change is acquired heterochromia, the development of different colored eyes or different colors within one eye, later in life. This condition often results from an underlying health issue or external factor affecting the iris’s pigment.
Certain medications are known to cause a lasting change in iris pigmentation. Prostaglandin analogs, prescribed as eye drops to treat glaucoma, can increase melanin production in the iris. This side effect typically causes lighter colored eyes (blue or green) to darken over time, shifting toward a brown hue.
Physical injury to the eye can also lead to a permanent color alteration. Trauma, such as a blunt force impact, can damage the iris tissue, causing pigment loss or structural changes. A severe injury may also cause a pupil to become permanently dilated (traumatic mydriasis), which can make the affected eye appear darker in comparison to the other.
Specific diseases can also cause permanent changes in eye color. Fuch’s heterochromic iridocyclitis, a chronic inflammation, often results in depigmentation of the affected iris, which can make a brown eye appear blue or gray. Conversely, pigment dispersion syndrome involves pigment granules scattering throughout the front of the eye, which can cause the iris to lighten or appear darker in certain areas.