Why Eyes Are Blue, Green or Brown: How Iris Colour Really Works

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How Human Eyes Get Their Colour: Melanin and Scattered Light
Zastavki

Human eyes come in only one real pigment: melanin, the same brown-black substance that colours skin and hair. Brown eyes have a lot of it in the front layer of the iris. Blue eyes have almost none there, and their blue comes from the way the tissue scatters light, much as the sky looks blue without containing any blue paint. Green, hazel and grey eyes sit in between, mixing a modest amount of pigment with that scattered light.

That simple idea explains a lot: why a newborn's blue eyes can turn brown, why blue eyes seem to change shade with the weather, and why there is no such thing as a truly "blue gene". Below, we look at how the iris is built, how each common eye colour forms, what genetics research has found, and which popular claims about eye colour do not hold up.

What the coloured part of the eye is made of

The iris is the ring of muscle and connective tissue that controls the size of the pupil. For eye colour, two layers matter.

  • The pigment epithelium is a thin sheet at the back of the iris. In almost everyone, whatever their eye colour, it is packed with dark melanin. Its job is to stop stray light from passing through the iris and blurring vision.
  • The stroma is the thicker front layer, a loose mesh of collagen fibres, blood vessels and pigment cells called melanocytes. This is where eye colours differ.

Interestingly, people with blue and brown eyes have roughly similar numbers of melanocytes in the iris. The difference lies in how much pigment those cells make and store. Researchers who study iris colour describe brown irises as containing more melanin and more of the tiny pigment packages, called melanosomes, than blue ones. Melanin comes in two forms: eumelanin, which is brown-black, and pheomelanin, which is red-yellow. The balance between them affects the warmer tones seen in some green and hazel eyes.

How each eye colour forms

Think of every eye colour as a recipe with two ingredients: how much melanin sits in the stroma, and how much light the stroma's fibres scatter back toward the viewer.

Brown

Brown is by far the most common eye colour worldwide. A brown iris has a dense layer of melanin in the stroma, which absorbs most of the light that enters. Very little escapes to be scattered, so we see the colour of the pigment itself, from light amber to almost black.

Hazel

Hazel eyes contain a moderate amount of melanin, often unevenly spread, with more pigment near the pupil. Some light is absorbed and some is scattered, so the eye shows a blend of brown, gold and green that can look different under different lighting.

Green

Green eyes have a small amount of pigment, often with a relatively high share of yellowish pheomelanin. The stroma scatters short blue wavelengths, and that scattered blue mixes with the yellow-brown pigment. The result reads as green, even though no green pigment exists in the eye. Green is one of the rarer colours globally and is most common in northern and central Europe.

Grey

Grey eyes are the least understood. Like blue eyes, they have very little melanin in the stroma. One leading explanation is that the stroma in grey eyes contains more or denser collagen, which scatters all wavelengths more evenly instead of favouring blue. That would give a pale, silvery look. Even writers who popularised this idea describe it as a current theory rather than settled fact.

Blue

Blue eyes have little or no melanin in the stroma. Light entering the iris passes into this nearly colourless layer, and the fine structure of the tissue scatters the short blue wavelengths back out more strongly than the longer red ones. The longer wavelengths travel deeper and are absorbed by the dark epithelium at the back. What reaches us is mostly blue.

Structural colour: blue without blue pigment

Colour that comes from the physical structure of a material, rather than from pigment, is called structural colour. It is behind the shimmer of a peacock's feathers, the blue of many butterfly wings and the iridescence of some beetles. Blue eyes are a simple, non-iridescent version of the same principle.

The physics is related to the Tyndall effect, the scattering of light by tiny particles suspended in a medium, and to Rayleigh scattering, which makes the daytime sky blue. In both cases, the scattering particles or structures are small compared with the wavelength of light, and short wavelengths are scattered more than long ones. The poetic comparison between blue eyes and the sky is, in a loose sense, scientifically fair.

Not every colour in nature works this way, though. Most of the colours we see in leaves, flowers, skin and hair come from pigments that absorb some wavelengths and reflect others. Structural colour is the exception, not the rule, which is part of what makes blue eyes interesting.

Because structural colour depends on light, blue and grey eyes can look noticeably different with lighting, the colour of clothing nearby, or the size of the pupil. When the pupil narrows in bright light, the iris tissue bunches up and can look lighter or more intense. The eye does not change colour, but our perception of it does.

The genes behind eye colour

School textbooks once taught eye colour as a simple dominant-and-recessive trait, with brown beating blue. Real inheritance is more complicated. Eye colour is polygenic, meaning many genes contribute, and the colours form a continuum rather than neat boxes.

The biggest single influence comes from a region on chromosome 15 containing two neighbouring genes, OCA2 and HERC2. OCA2 helps melanocytes produce and store melanin. A 2008 study by Manfred Kayser and colleagues, using genome-wide association studies in Dutch populations, identified HERC2 as an independent iris colour gene. Later work showed that a common variant within HERC2 acts like a dimmer switch for OCA2, reducing melanin production in the iris and strongly associated with blue eyes in Europeans. Other genes, including some involved in skin and hair colour, fine-tune the shade.

One practical consequence: two blue-eyed parents usually have blue-eyed children, but not always. Because several genes are involved, unexpected combinations happen and do not, on their own, say anything about parentage.

When eye colour changes

Eye colour is fairly stable in adults, but it does change at certain stages of life, and occasionally a change is a sign worth checking.

  • Babies. Many babies of European descent are born with blue or blue-grey eyes because the melanocytes in the stroma have not yet produced much pigment. Babies of other backgrounds are often born with slate-grey or brown eyes. The final colour usually settles during the first months of life, although subtle shifts can continue for longer.
  • Two colours at once. Heterochromia means two different coloured eyes, or patches of a different colour within one iris. It is often harmless and inherited, but it can be linked to certain conditions, such as Horner syndrome or Waardenburg syndrome, especially when it appears in infancy.
  • Medicines. Some eye drops used to treat glaucoma can gradually darken a light iris by increasing pigment.
  • A new change in an adult. If one eye changes colour, a spot appears in the iris, or a change comes with pain, redness or blurred vision, talk to a doctor or an eye specialist. Most changes are harmless, but some need an examination.

Common myths about eye colour

  • "Blue eyes contain blue pigment." They do not. Blue is created by scattered light in a nearly pigment-free stroma.
  • "Eye colour is controlled by one gene." Many genes are involved, with OCA2 and HERC2 having the largest effect.
  • "Brown-eyed parents can never have a blue-eyed child." They can, if both carry variants associated with light eyes.
  • "Your eye colour reveals your personality or health." There is no good evidence that eye colour predicts character. The iris is also not a map of the body's organs, despite the claims of iridology.
  • "Violet eyes are a natural colour." Eyes described as violet are generally very light blue or blue-grey eyes seen in particular lighting, sometimes enhanced by makeup or photo editing.

Next time you look closely at someone's eyes, you are seeing two things at once: a pigment made by living cells and a small optical effect created by the structure of the tissue. In blue eyes, the colour is almost entirely the second.

Sources: Paul Van Slembrouck, "Structural eye color is amazing", Medium; Kayser et al. 2008, Three genome-wide association studies and a linkage analysis identify HERC2 as a human iris color gene, American Journal of Human Genetics; Eye (Nature), review of iris colour, melanin and heterochromia; originally inspired by an article by Anna LeMind of Learning Mind via The Mind Unleashed.

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