Eye color is a complex, physical trait determined by multiple genes that influence pigment production in the eye’s iris. While the final shade is genetically determined, the color of the eyes is not always fixed immediately after birth. The change from a light color, such as blue, to green is a common and natural occurrence during the first few years of life. However, any permanent shift in eye color in adulthood is extremely rare and often signals an underlying medical change that requires professional attention.
The Mechanism of Blue and Green Eyes
The colors perceived in the human iris are not due to blue or green pigments existing within the tissue itself. Eye color is an optical effect created by the interaction of light with the amount of the brown pigment melanin present in the iris stroma. Blue eyes contain very low concentrations of melanin in the front layer of the iris.
When light enters an eye with low melanin, the longer wavelengths are absorbed by the dark tissue at the back of the iris. The shorter, blue wavelengths are scattered back out by the dense, turbid tissue of the stroma, a phenomenon known as Rayleigh scattering. This process is identical to what makes the sky appear blue.
Green eyes possess slightly more melanin than blue eyes. This low to moderate concentration of melanin in the stroma often contains a yellowish or amber hue. When the scattered blue light mixes with this small amount of yellow pigment, the result is the appearance of green.
Developmental Changes During Infancy
The natural transition from blue to green eye color occurs almost exclusively during infancy and early childhood. Most babies, particularly those of European descent, are born with blue or slate-gray eyes because the specialized cells that produce melanin, called melanocytes, are relatively inactive at birth. They have not yet been exposed to enough light to stimulate full melanin production.
As the infant is exposed to light over the first months of life, the melanocytes begin to mature and consistently produce melanin. If a child is genetically predisposed to have green eyes, this is the time when the slight increase in melanin causes the visible color change. The darkening process usually begins between six and twelve months of age, though it can continue more subtly for up to three years.
Once the melanocytes reach their genetically predetermined level of melanin production, the eye color stabilizes. The change reflects the child’s inherited genetic coding for a lighter eye color that required post-natal melanin development. This developmental window is when a natural, physiological blue-to-green color shift occurs.
Changes in Adulthood: Medical and Perceptual Factors
After the initial developmental window closes, the physiological color of the iris rarely changes. Any noticeable shift is typically due to either perceptual illusion or a medical issue, making the color appear to change without biological alteration to the iris. External lighting conditions, such as natural sunlight versus indoor fluorescent bulbs, can dramatically affect how the light scattering appears to an observer.
The color of clothing, makeup, or surrounding environments can also reflect into the iris, creating a temporary optical effect that makes blue eyes seem greener. Changes in pupil size, often triggered by emotional responses or shifts in light levels, can alter the eye’s appearance. When the pupil dilates, the dark color of the interior eye contrasts with the iris, sometimes making a light blue or hazel eye appear deeper or more green.
In rare instances, a permanent, physiological change in adult eye color, especially one that is unilateral (affecting only one eye), requires medical attention. Certain types of inflammation, such as Fuch’s heterochromic iridocyclitis, can cause a gradual loss of pigment in the affected iris, leading to a lightening or change in hue. Trauma to the eye can also damage the iris tissue, resulting in a permanent color shift.
Specific medications are also known to cause genuine changes in iris pigmentation. Prostaglandin analogs, a class of medication used to treat glaucoma, can increase the concentration of melanin in the iris, often causing eyes to darken. This change is typically gradual and can be permanent, showing that melanin concentration can still be influenced by external chemical factors after childhood.