Eye Colour Explained: How Genetics, Iris Pigment and Light Create Different Shades

Why your eyes are far more remarkable than you might imagine

Have you seen the studios that have appeared in some continental cities in recent years? They take a photo of your eye, carry out some clever technical whatnots and produce a poster of your iris. They can combine the image of yours with that of a loved one which seems to be popular.  They really are quite striking and I’m not sure why they haven’t caught on here. Seeing the iris blown up you get a sense of just how intricate and individual they are.

Perhaps you’ve looked in the mirror and wondered why your own eye colour seems to change in different lighting, or why your children have different coloured eyes from you. Eye colour is one of the most fascinating features of the human body.

Most of us think of our eyes simply as blue, brown, green or hazel, but the science behind eye colour is far more complex than that. The colour of your iris is influenced by genetics, melanin, the way light is scattered, and even the microscopic structure of the iris itself. Every pair of eyes tells a unique story, and no two irises are exactly alike.

As optometrists, we spend our days examining people’s eyes in incredible detail. We see a very wide range of different shades of eye colour every week, yet patients are often surprised to learn that the colour they see isn’t always created by pigment alone.

So, what determines eye colour? Why are some eye colours rarer than others? Can eye colour change? And is it really true that everyone with blue eyes shares a common ancestor?

Let’s take a closer look.

Why are we so fascinated by eye colour?

There is something about another person’s gaze that immediately captures our attention. Long before we notice someone’s hairstyle or clothing, we often notice their eyes.

• Artists have painted them.
• Photographers use them as the focal point of portraits.
• Poets have written about them for centuries.
• Even today, social media platforms such as Instagram are full of close-up photographs celebrating the incredible colours and patterns found within the human iris.

Yet despite this fascination, many people know surprisingly little about what actually creates eye colour.

The truth is that the colour of your eyes isn’t simply painted onto the iris like paint on a wall. Instead, it is the result of a remarkable combination of genetics, pigmentation, light, reflection and microscopic anatomy.

What part of the eye gives it its colour?

The iris is a circular ring of living tissue surrounding the pupil. Its primary job isn’t actually to provide colour at all. Instead, it acts rather like the aperture of a camera, constantly adjusting the size of the pupil to control how much light reaches the retina at the back of the eye.

In bright sunshine, the pupil becomes smaller.

In dim lighting, it enlarges.

This happens automatically thousands of times every day.

The iris itself contains muscles, blood vessels, connective tissue and specialised pigment cells. Under magnification it resembles an intricate landscape filled with tiny ridges, crypts and fibres. In fact, these patterns are so individual that iris recognition is now one of the most accurate forms of biometric identification in the world.

Not even identical twins have identical irises.

What determines the colour of your eyes?

If there is one word that explains eye colour more than any other, it is melanin.

Melanin is the natural pigment that also gives colour to our skin and hair. The amount and distribution of melanin within the iris largely determine whether someone’s eyes appear brown, hazel, green, blue, grey or amber.

Generally speaking:

• Large concentrations of melanin produce dark brown eyes.
• Moderate amounts of melanin create lighter brown or hazel eyes.
• Small amounts of melanin contribute to green eyes.
• Very little melanin results in blue or grey eyes.

However, this is only part of the story.

The final shade depends not only on the amount of pigment present but also on how light interacts with the microscopic layers of the iris. Reflection, contrast, brightness and the internal structure of the iris all influence the hue we perceive.

This is why eye colour often appears to change depending on the lighting, clothing or even the size of the pupil.

The remarkable genetics behind eye colour

Perhaps the most fascinating discovery in recent years concerns the genetics of eye colour.

For many years, children were taught that eye colour was determined by a simple dominant and recessive gene. Brown was dominant, blue was recessive, and predicting eye colour seemed straightforward.

Modern genetics has shown that reality is far more interesting.

Scientists now know that eye colour is influenced by numerous genes, with the OCA2 and HERC2 genes playing particularly important roles in regulating melanin production within the iris. Rather than acting as a simple on-off switch, these genes work together with several others to produce the wide spectrum of eye colours seen today. (PubMed)

This explains why brothers and sisters can have completely different eye colours and why predicting a baby’s eye colour is often much more difficult than people expect.

Did everyone once have brown eyes?

Here’s a fact that almost always surprises patients.

Current evidence suggests that all humans originally had brown eyes.

Between approximately 6,000 and 10,000 years ago, a genetic mutation occurred in a regulatory region associated with the HERC2 and OCA2 genes. Rather than creating a new blue pigment, this mutation reduced the amount of melanin produced within the iris.

The result was the world’s first blue eyes. (EurekAlert!)

Even more remarkably, genetic studies suggest that everyone with naturally blue eyes today inherited this same ancient genetic variation, meaning they almost certainly share a distant common ancestor who lived thousands of years ago. (EurekAlert!)

That doesn’t mean every blue-eyed person is closely related, of course, but it does illustrate just how recently blue eyes appeared in human history.

Over thousands of generations, additional genetic variation created today’s beautiful spectrum of blue, green, hazel, grey and amber eyes.

Blue eyes aren’t actually blue

This is probably my favourite fact about eye colour.

Blue eyes contain very little blue pigment.

Instead, they appear blue because of the way light is scattered within the layers of the iris.

Exactly the same principle explains why the sky appears blue.

Sunlight contains every colour of the visible spectrum. Shorter blue wavelengths are scattered more readily than longer red wavelengths, creating the familiar blue appearance.

The same optical phenomenon occurs within blue irises.

So if you’ve always assumed your blue eyes contained blue pigment, you’re in good company—but the science tells a rather different story.

Brown eyes: the world’s most common eye colour

Brown eyes are by far the most common eye colour worldwide, with an estimated 70–80% of people having some shade of brown.

Their colour comes from higher concentrations of melanin within the iris. Brown eyes vary enormously. Some are almost black, others contain warm honey tones, golden flecks or rich copper highlights that become visible only in bright sunlight. No two brown irises are identical.

Hazel, green, grey and amber eyes

Hazel eyes are particularly fascinating because they often appear to change colour.

This isn’t because the iris is changing, but because varying amounts of pigment combined with different lighting conditions alter how we perceive the eye.

Hazel eyes may appear green outdoors, golden indoors and brown in low light.

Green eyes are among the rarest natural eye colours, occurring in only around two per cent of the world’s population.

Grey eyes are rarer still and result from unique combinations of iris structure and pigmentation.

Amber eyes have a distinctive golden or copper appearance caused by particular pigment concentrations rather than mixed colours.

Together these eye colours create the extraordinary diversity we see in clinics every day.

Can your eye colour change?

This is another question patients ask regularly. The answer is yes—but usually only to a limited extent.

Babies often have blue or grey eyes at birth because relatively little melanin has developed within the iris. During the first year or two of life, increasing pigmentation may cause the eyes to become brown, hazel or green.

Adults also notice changes. Most are simply the result of different lighting, changes in pupil size or the colours we wear. However, genuine changes affecting one eye alone should always be assessed by an optometrist or ophthalmologist.

When should changes in eye colour be investigated?

Although most variations are harmless, there are occasions when changes deserve attention.

These include:

• one eye becoming noticeably darker or lighter
• new areas of pigmentation
• changes following an eye injury
• differences developing between the two eyes
• associated pain or blurred vision

Fortunately, these situations are uncommon, but they illustrate why routine eye examinations are about much more than simply updating glasses.

Different coloured eyes: heterochromia

Some individuals naturally have two different coloured eyes. This condition is known as heterochromia. Sometimes one eye is blue and the other brown. In other cases, only part of one iris differs in colour.

Most cases are completely harmless and simply reflect differences in melanin distribution during development. Occasionally, however, heterochromia develops later in life following injury or disease and should be examined.

What about albinism?

Albinism affects the production of melanin throughout the body. People with albinism often have very light skin, hair and eye colour because their bodies produce reduced amounts of pigment.

Their irises may appear pale blue, grey or even slightly pink because the lack of pigmentation allows underlying blood vessels to become more visible. Albinism can also affect vision, making regular eye care particularly important.

Does eye colour affect vision?

For the vast majority of people, eye colour has very little effect on vision.

Whether your eyes are blue, brown, green, hazel or amber, the sharpness of your sight depends far more on the health of your cornea, lens, retina and optic nerve than on the colour of your iris.

However, people with lighter eyes sometimes report greater sensitivity to bright sunlight because there is less melanin available to absorb scattered light.

Good quality sunglasses remain beneficial whatever colour your eyes happen to be.

Looking beyond colour

As optometrists, we certainly notice eye colour—but we’re interested in much more than that.

During a comprehensive eye examination we examine the iris, pupil, lens, retina and blood vessels in detail.

We look for early signs of cataracts, glaucoma, diabetes, hypertension and many other conditions.

The beautiful colours of the iris may be what catches our attention initially, but the health information contained within the eye is even more remarkable.

Every pair of eyes is unique

Perhaps the most wonderful fact of all is that no two pairs of eyes are exactly alike.

The intricate combination of pigment, iris fibres, rings, crypts and microscopic markings creates a pattern unique to every individual.

Your eye colour is only one part of that story. Whether your eyes are deep brown, sparkling blue, hazel, green, grey or amber, they are unlike anyone else’s in the world.

Frequently Asked Questions

1. What determines the colour of your eyes?

Eye colour is determined primarily by the amount and distribution of melanin within the iris, together with the way light interacts with its microscopic structure. Genetics plays the biggest role, with several genes influencing the final result.

2. How many eye colours are there?

Although we commonly describe eye colours as brown, blue, green, hazel, grey and amber, there is actually a continuous spectrum of shades and hues. Every iris has its own unique combination of pigmentation and pattern.

3. Can eye colour change over time?

Yes. Babies’ eyes often change colour during early childhood as melanin develops. Adults may notice apparent changes due to lighting or pupil size, while genuine changes affecting one eye should be examined.

4. Are certain eye colours rarer than others?

Yes. Brown is the most common eye colour worldwide. Green, grey and amber are much rarer, with green eyes occurring in only around two per cent of the world’s population.

5. Does eye colour affect vision?

Not significantly. Eye colour itself has little influence on visual sharpness, although people with lighter eyes may notice greater sensitivity to bright sunlight.

6. Can eye colour be changed naturally?

No. The underlying genetics and melanin concentrations within the iris determine natural eye colour. Lighting and clothing may alter its appearance, but they do not permanently change it.

7. What genes influence eye colour?

Many genes contribute, but OCA2 and HERC2 are among the most important. These genes regulate melanin production and help determine the amount of pigment within the iris. (PubMed)

8. Are blue eyes more common in specific regions?

Yes. Blue eyes are particularly common in Northern and Eastern Europe because the genetic mutation responsible became more prevalent there over thousands of years. (PubMed Central (PMC))

9. Can contact lenses permanently change eye colour?

No. Coloured contact lenses simply alter the appearance of the iris while they are being worn. Once removed, your natural eye colour remains unchanged.

10. Do babies have the same eye colour as their parents?

Not necessarily. Eye colour is influenced by multiple genes inherited from both parents and earlier generations. This means children can have eye colours different from either parent, particularly when family genetics include a variety of eye colours.

Final thoughts

Eye colour is far more than a cosmetic feature. It represents a remarkable combination of genetics, biology, optics and evolution. From the ancient mutation that gave rise to blue eyes to the intricate patterns that make every iris unique, our eyes tell a fascinating story about who we are and where we came from.

At Bellamy Eyecare, we believe understanding your eyes is just as important as looking after them. During every comprehensive eye examination, we’re always happy to answer questions—not only about your vision, but also about the remarkable science behind your eyes.