Why blue eyes aren’t really blue — and what makes green eyes so rare

Scientists say the familiar colours of human irises result less from pigment than from the way light is scattered and absorbed in the eye, explaining why blue eyes are not truly blue and why green eyes remain rare.

Researchers, citing the physics of light and the biology of melanin, say the blue appearance of some irises arises from the Tyndall effect — the same process that makes the sky and ocean look blue — rather than from blue pigment in the iris. Brown eyes, by contrast, contain higher concentrations of melanin, which absorb more light and produce a darker appearance.

Dr. Davinia Beaver of Bond University, writing in The Conversation, explained that "blue eyes contain very little melanin. In blue eyes, the shorter wavelengths of light — such as blue — are scattered more effectively than longer wavelengths like red or yellow." With less melanin to absorb light, the scattered short-wavelength light dominates what observers perceive as a blue hue.

Green eyes, which are uncommon worldwide, reflect an intermediate amount of melanin: enough pigment to alter scattering but not so much as to appear brown. Hazel and mixed-colour irises result from uneven melanin distribution in the iris stroma, producing a mosaic of colours that can appear to shift under different lighting conditions or against different backgrounds.

Longstanding assumptions that a single gene controls eye colour have been revised in light of genomic studies. Multiple genes influence the amount, type and distribution of melanin in the iris, which helps explain why siblings can have markedly different eye colours and why two parents with blue eyes can occasionally have a child with green or light brown eyes.

The developmental timeline of iris colour also reflects melanin production. Many babies, particularly those of European ancestry, are born with blue or grey eyes because melanin levels in the iris are low at birth. Pigment typically increases during the first few years of life, and eye colour can shift from blue to green or brown as melanin accumulates. By adulthood, eye colour is generally stable, though slight variation with lighting, age or certain medical conditions is possible.

Medical and traumatic causes can change appearance as well. Heterochromia — a condition in which the two eyes differ in colour — may be congenital, arise from injury, or be associated with certain health conditions. Some well-known public figures, such as actors Kate Bosworth and Mila Kunis, have forms of heterochromia. Musician David Bowie’s eyes appeared different in colour because an accident left one pupil permanently dilated, creating the illusion of two different-coloured irises.

Scientists also note that perception plays a role: clothing, makeup and ambient light can influence how an eye colour is judged. Geographical patterns remain evident: blue eyes are more common in parts of northern Europe and are currently the most frequently reported eye colour in the United Kingdom, while brown eyes are the most common worldwide.

The pupil, the black central opening of the iris, affects the appearance of the iris by controlling how much light enters the eye. Muscles in the iris constrict or dilate the pupil in response to light levels and other stimuli; in bright conditions the pupil constricts to limit incoming light, and in low light it dilates to admit more. The pupil appears black because light that passes through it is absorbed by the retina and not reflected back.

These findings unify physics and genetics to clarify everyday observations about eye colour while underscoring that what people see as a pigment is often the result of structural optics and variable melanin levels influenced by multiple genes.