The iris, the colored ring surrounding the pupil, is responsible for controlling the amount of light entering the eye by adjusting the pupil’s size. The wide spectrum of human eye colors results from two primary factors: the total amount of melanin present in the iris tissue and how light interacts with the iris structure. Melanin, the same pigment that determines hair and skin color, is the sole pigment in the human iris, and its concentration dictates the final appearance.
The Primary Spectrum of Eye Colors
Brown is the most common eye color globally, accounting for approximately 79% of the world’s population. This color ranges from light sepia to deep, dark brown and is prevalent across all continents.
Blue eyes are the second most common, found in about 8% to 10% of people worldwide, and are most prevalent in Europe. This pale, uniform hue is not caused by blue pigment.
Hazel eyes are a dynamic color appearing as a mix of brown, green, and gold, often seeming to change depending on the lighting. They are found in about 5% of the global population.
Green eyes are the least common of the primary colors, appearing in only about 2% of the world’s population. This color typically has a more uniform appearance than hazel and is most often seen in people of Northern and Central European descent.
How Melanin and Light Scattering Create Color
Melanin is the only pigment found in the human iris, and it is a dark brown substance. The concentration of this pigment within the stroma, the front layer of the iris, determines the eye color. High concentrations of melanin cause the iris to absorb most incoming light, resulting in brown eyes.
Lighter eye colors, such as blue, green, and gray, are structural colors that rely on physics. In irises with very low amounts of melanin, light is not fully absorbed and encounters the stroma. This causes a phenomenon called Rayleigh scattering, which preferentially scatters the shorter, blue wavelengths of light back out of the eye. This makes the iris appear blue, similar to how the sky appears blue.
Green and hazel eyes result from an intermediate level of melanin combined with this scattering effect. For green eyes, a small amount of yellow pigment is present in the stroma. When the scattered blue light combines with this yellow pigment, the eye perceives the color as green. Hazel eyes contain a moderate amount of melanin, often distributed unevenly, creating a mix of brown, gold, and green.
Rare and Uncommon Eye Color Variations
Beyond the primary colors, a few rare shades and conditions create unique iris appearances. Gray eyes are similar to blue eyes, possessing very low melanin, but they are distinct due to a different density of collagen in the stroma. This structural difference causes light scattering to produce a muted, silvery hue.
Amber eyes are a solid, uniform color with a golden or coppery tint, often confused with hazel eyes. This color is caused by the presence of a yellow pigment called lipochrome, which is distributed evenly throughout the iris. Unlike hazel, amber eyes do not contain the flecks of brown or green.
Heterochromia is a condition where a person has two different colored eyes (complete heterochromia) or patches of a different color within a single iris (sectoral heterochromia). This variation is due to an uneven distribution or lack of melanin.
For individuals with a severe lack of pigment, such as in some forms of albinism, the iris may appear red or pink. This occurs because the near-total absence of melanin allows red light reflecting off the blood vessels at the back of the eye to show through. The “violet” eye color is typically an optical illusion resulting from the combination of this underlying redness and the deep blue structural color caused by minimal pigment.
Developmental Changes in Iris Color
Many infants of European descent are born with blue or grayish eyes due to a temporary lack of pigment. Melanocytes, the cells that produce melanin, do not begin full production until after birth when the eyes are exposed to light. As melanocytes become active, the baby’s eye color may change, typically darkening over the first 6 to 12 months.
The final eye color is usually established by age three, though minor changes can continue into later childhood. Once a person reaches adulthood, the iris color is generally stable. Any sudden or significant change in adult eye color is uncommon and should be evaluated by a medical professional. Such changes can sometimes be an indicator of an underlying medical condition, trauma, or the side effects of certain medications.