Eye color is typically established and fixed after early infancy, but the perception of that color can be highly fluid and dynamic. When people observe a shift in their green eyes, it is most often an illusion based on the environment, though a true, permanent change can be a sign of underlying medical issues.
How the Eye Produces Green Color
The green color in the human eye is not caused by a green pigment, but by a specific combination of moderate pigmentation and light scattering within the iris. The color of the iris is primarily determined by the concentration and distribution of melanin, the same brown pigment that colors skin and hair. People with green eyes have a moderate concentration of melanin in the iris stroma, which is the front layer of the iris.
When light enters the eye, the shorter blue wavelengths are selectively scattered by the turbid stroma, a phenomenon known as Rayleigh scattering. This scattered blue light then mixes with the yellowish-brown pigment present in the stroma, producing the visible green color. Green eyes are considered structural colors because their appearance relies on how light physically interacts with the iris tissue.
Factors That Shift Color Perception
The appearance of green eyes can shift dramatically, a direct consequence of their reliance on light scattering. The most significant factor influencing this temporary change is the quality and intensity of ambient light. Different light sources, such as bright sunlight or warm artificial light, alter how light reflects and scatters within the iris, making the color seem lighter or darker.
The colors worn near the face can also create an optical illusion that subtly alters the eye color. Wearing a green or blue shirt, for example, can cause the eye to reflect that color, intensifying the perceived green hue through visual contrast. This phenomenon manipulates how the surrounding light is reflected back to the observer without changing the actual pigment.
The size of the pupil, the black center of the eye, also plays a role in color perception. When the pupil dilates in dim light, the colored part of the iris is compressed and appears darker. Conversely, when the pupil constricts in bright light, the iris expands, often making the color look lighter.
Medical Conditions Linked to Eye Color Change
A true, lasting change in adult eye color is rare and typically signals a physiological change that warrants an examination by an ophthalmologist. One such condition is acquired heterochromia, where a person develops a difference in color between their two eyes or distinct color patches within one iris later in life. This is often a sign of an underlying disease or trauma to the eye.
Certain medications, particularly prostaglandin analogs used to treat glaucoma, can cause a permanent darkening of the iris. These drops stimulate the production of melanin in the stroma, causing lighter eyes to gradually turn a shade of brown. This effect is a true pigmentary change, though it is usually harmless.
Rare inflammatory conditions can also lead to color shifts. Fuchs’ heterochromic iridocyclitis is a chronic inflammation that can cause the affected iris to lose pigment, resulting in a lighter color over time. Another serious cause of darkening is iris melanoma, a rare form of cancer involving the growth of pigmented cells. Any permanent or sudden change in eye color should be evaluated quickly by an eye care professional.