Eye color is a complex trait determined by the concentration of melanin in the iris and the physical phenomenon of light scattering. The different shades observed in the human eye, including green and hazel, exist along a spectrum of melanin levels. Understanding the biological mechanisms that create these colors clarifies the visual and physical differences between them.
The Biology of Eye Color
The color perceived in the iris is governed by the amount of melanin, specifically eumelanin, located within the iris’s front layer, the stroma. Unlike brown eyes, which have a high concentration of melanin, lighter eye colors like blue, green, and hazel have low to moderate amounts of this pigment. Because there are no actual blue or green pigments in the human iris, these colors arise from an optical effect.
The stroma’s low melanin content allows light to enter and scatter, a process similar to Rayleigh scattering. This scattered blue light combines with any trace of yellowish pigment (lipochrome) present in the stroma. The resulting color is a blend of this structural blue and the yellowish pigment, creating the green or hazel hue. The specific concentration and distribution of melanin determine which of these lighter colors manifests.
Defining Green Eyes
Green eyes are characterized by a low, consistent concentration of melanin across the entire iris. This level of pigment is slightly higher than in blue eyes but lower than in hazel eyes. The uniform green color results from the stroma’s scattered blue light interacting with a small, uniform amount of yellowish lipochrome pigment.
This balance of low melanin and light scattering leads to a stable, single hue that ranges from light jade to deep emerald. Green eyes are the rarest natural eye color, occurring in approximately two percent of the world’s population. Their distinguishing feature is the homogeneity of the color, maintained from the pupil to the outer edge of the iris.
Defining Hazel Eyes
Hazel eyes feature a moderate concentration of melanin, placing them between green and light brown eyes. The defining characteristic of hazel is its multi-tonal nature, involving an uneven distribution of pigment across the stroma. This creates a blend of colors, most commonly brown, gold, and green.
The uneven pigmentation often manifests as a distinct central ring of brown or gold around the pupil, where melanin concentration is highest. This then transitions into an outer ring of green or amber. This variable distribution means that hazel eyes are a complex, multi-zoned composition, reflecting the blend of brown and green tones found in a hazelnut.
Visual Distinctions and Common Misconceptions
The most straightforward way to distinguish between the two colors is by examining the uniformity of the iris. A green eye maintains a single, stable hue across the entire surface, with minimal variation in the pigment pattern. In contrast, a hazel eye clearly displays two or more distinct color zones, usually a ring of brown or gold near the center transitioning to green or amber toward the periphery.
A common misconception is that hazel eyes change color depending on a person’s mood. The color of the iris pigment is constant, but the multi-zoned pigmentation makes hazel eyes highly reactive to external factors. The appearance of shifting color is an optical illusion caused by changes in surrounding light, clothing color, or makeup, all of which alter how light interacts with the various pigment zones.