The condition is called heterochromia, or more formally heterochromia iridis (referring to the iris). It describes any situation where a person’s eyes are not a single uniform color, whether that means one blue eye and one green eye, a ring of gold around a blue pupil, or a wedge of brown cutting through an otherwise green iris. Most cases are congenital, harmless, and simply the result of uneven melanin distribution during fetal development, but the term also covers color differences that show up later in life due to injury, disease, or medication.
The Three Types of Heterochromia
Heterochromia is not one look. It comes in three distinct forms, each with a different appearance and, sometimes, a different underlying cause.
- Complete heterochromia: Each eye is a clearly different color. One iris might be blue while the other is green or brown. This is the most recognizable version and the one most people picture when they hear the word.
- Sectoral heterochromia: Part of one iris is a different color from the rest of that same iris. It often looks like a wedge or patch of brown in an otherwise blue or green eye. Sectoral heterochromia frequently results from benign genetic mosaicism or a somatic mutation that occurs during embryonic development.1PubMed Central. Artistic Iris: A Case of Congenital Sectoral Heterochromia Iridis
- Central heterochromia: The area closest to the pupil is one color while the outer ring of the iris is another. A common example is a copper or gold ring around the pupil surrounded by blue or green. This is the subtlest form and sometimes goes unnoticed unless someone looks closely in good light.
When someone says they have “blue and green eyes,” they could mean complete heterochromia with one of each, or they could be describing central or sectoral heterochromia where both colors appear within the same iris. The distinction matters medically because complete heterochromia in particular can occasionally signal an underlying condition, while central heterochromia is almost always a harmless quirk of pigmentation.
Why Eyes Have Color Differences at All
A common misconception is that blue eyes contain blue pigment and brown eyes contain brown pigment. In reality, human irises contain only one pigment: melanin. What changes from person to person is not which pigment is present but how much melanin the iris cells produce and how it is packaged. Blue eyes have relatively little melanin in the front layer of the iris, so light scatters in a way that looks blue (similar to why the sky appears blue). Brown eyes have dense melanin that absorbs most wavelengths. Green eyes sit somewhere in between, with moderate melanin plus some light-scattering structural effects.
Researchers confirmed decades ago that the number of melanocytes (the cells that make melanin) in the iris is roughly the same regardless of eye color. Across blue, green-hazel, and brown eyes, the average melanocyte count does not differ in a statistically meaningful way.2PubMed. Melanocytes and iris color. Light microscopic findings What does change is the size and number of melanin-containing structures inside those cells. Blue irises have far fewer and smaller melanosomes compared to brown irises, a difference that is large and statistically clear.3PubMed. Melanocytes and iris color. Electron microscopic findings
This means heterochromia is not about having two different pigments side by side. It is about some melanocytes producing or packaging melanin differently from their neighbors. In complete heterochromia, one whole iris developed with a different melanin load than the other. In sectoral heterochromia, a cluster of cells in one part of a single iris ended up on a different program. In central heterochromia, the inner and outer rings of the iris simply matured with slightly different melanin concentrations.
The Genetic Landscape Behind Eye Color
Eye color is not controlled by a single gene. The strongest single predictor is a variant in the HERC2/OCA2 region on chromosome 15, which plays a large role in distinguishing blue from brown eyes.4PubMed Central. Association between Variants in the OCA2-HERC2 Region and Blue Eye Colour in HERC2 rs12913832 AA and AG Individuals But eye color is genuinely polygenic: dozens of other genetic variants nudge the final shade. A specific missense variant in OCA2 has been linked to green and hazel eyes in European populations.5PubMed Central. A global view of the OCA2-HERC2 region and pigmentation This genomic region is highly conserved across vertebrates, suggesting it has been under strong evolutionary pressure for a long time.6PubMed Central. The Chromatin Organization Close to SNP rs12913832, Involved in Eye Color Variation, Is Evolutionary Conserved in Vertebrates
So where does heterochromia fit into this genetic picture? In most congenital cases, it is not the result of inheriting a specific “heterochromia gene.” Instead, it arises from somatic events: a random mutation in one cell during early embryonic development, or a difference in how gene expression unfolds in one patch of tissue versus another. This is genetic mosaicism. Because the change happens after conception, it shows up in only some cells of the body, which is why only one eye or one section of an iris looks different. It is more of a developmental accident than a heritable trait, which is why heterochromia does not reliably run in families even though eye color itself does.
Congenital Heterochromia and Neural Crest Development
The melanocytes in your iris originally come from a group of embryonic cells called the neural crest. These cells migrate from the developing spinal cord to many destinations in the body, including the skin, parts of the gut, and the eyes. If something disrupts that migration or the subsequent development of melanocytes in one eye, the result can be heterochromia.
This shared origin explains why iris color differences sometimes appear alongside conditions affecting other neural-crest-derived tissues. Sectoral heterochromia, for example, has been reported in association with Hirschsprung’s disease, a condition where nerve cells are missing from parts of the large intestine. Both the eye finding and the intestinal finding reflect abnormalities in tissues that trace back to the neural crest.7PubMed. Iris sector heterochromia as a marker for neural crest disease This does not mean everyone with a patch of different color in their iris has a gut problem. Most do not. But the connection illustrates why doctors sometimes take note of heterochromia in newborns: it can be a soft marker worth following up on.
When Heterochromia Points to a Medical Condition
The vast majority of people with different-colored eyes have benign congenital heterochromia and need no treatment. But there are a handful of conditions where heterochromia is part of a larger clinical picture, and recognizing the difference matters.
Waardenburg Syndrome
Waardenburg syndrome is a genetic condition that affects melanocyte development broadly. It occurs in roughly one in 40,000 people and can cause a combination of hearing loss, patches of white hair or skin, and iris color abnormalities including heterochromia.8PubMed Central. Association of Waardenburg syndrome with a new mutation in the PAX3 gene: A case report and literature review It is classified into four subtypes based on which genes are mutated and which additional symptoms appear. PAX3 mutations are linked to types 1 and 3, while SOX10 mutations cause types 2 and 4.9PubMed. Functional analysis of Waardenburg syndrome-associated PAX3 and SOX10 mutations: report of a dominant-negative SOX10 mutation in Waardenburg syndrome type II The key clinical clue is that the eye color difference is rarely the only sign. If a child has brilliant blue eyes (or one very pale blue eye) along with a white forelock of hair or fails a newborn hearing screen, Waardenburg syndrome becomes a strong possibility.
Horner Syndrome
Horner syndrome involves disruption of the nerve pathway running from the brain to one side of the face. When it occurs at birth or early in life, the affected eye may end up lighter in color than the other because normal sympathetic nerve signaling helps stimulate melanin production in the iris. The classic trio of signs is a smaller pupil, a drooping eyelid, and a lighter-colored iris on the same side. One published case described a patient with two different eye colors and a constricted pupil in the lighter eye, who was ultimately diagnosed with congenital Horner syndrome.10PubMed Central. Heterochromia Unlike benign heterochromia, Horner syndrome points to a problem somewhere along the sympathetic nerve chain and usually warrants imaging to rule out a mass or injury.
Fuchs Heterochromic Iridocyclitis
Fuchs heterochromic iridocyclitis is a chronic, usually one-sided inflammation of the iris and ciliary body that gradually changes the color of the affected eye. Over time it can lead to cataracts, glaucoma, and sometimes clouding of the vitreous gel inside the eye.11PubMed Central. Treatment of Complications Due to Fuchs Heterochromic Iridocyclitis (FHI) – a Case Report The exact trigger remains uncertain, though researchers have proposed theories ranging from toxoplasma infection to chronic herpes virus activity. The heterochromia in Fuchs is acquired rather than congenital, so it tends to appear in young adulthood. If someone notices one eye slowly becoming a different shade and also experiences blurred vision or light sensitivity, this is a condition worth investigating.
Acquired Heterochromia From Injury or Medication
Not all heterochromia is present from birth. The iris can change color over the course of a lifetime, and some of the causes are surprisingly mundane.
Metallic Foreign Bodies and Siderosis
When a small iron-containing foreign body lodges inside the eye after an injury, the iron slowly oxidizes and deposits rust-colored pigment in surrounding tissues, including the iris. This process, called siderosis bulbi, can turn a blue or green iris noticeably brown on the injured side. In one documented case, a patient presented a full year after a penetrating injury with reduced vision and a visible change in iris color, ultimately traced to a retained ferrous foreign body.12PubMed Central. Siderosis bulbi as a consequence of a missed intraocular foreign body The color change is essentially a rust stain, and it signals that the foreign body needs to be removed before further damage occurs.
Glaucoma Eye Drops
Prostaglandin analogs like latanoprost, commonly prescribed for glaucoma, can darken the treated eye over months of use. In one study of patients using latanoprost in a single eye, about 70% developed a noticeable color difference between their treated and untreated eyes. Half of those showed a granular increase in surface pigmentation, while the other half showed a deeper stromal darkening.13PubMed Central. Incidence of iris colour change in latanoprost treated eyes The effect is more obvious in people who start with lighter or mixed-color irises, particularly hazel or green-brown eyes, because there is more room for the color to shift. It is usually irreversible, so patients prescribed these drops in just one eye are typically warned to expect some degree of asymmetry.
Should You See a Doctor About Heterochromia?
If you have had two different eye colors for as long as you can remember and there are no other symptoms, the answer is almost certainly no. Congenital heterochromia that exists in isolation is a cosmetic curiosity, not a disease. Many people go their whole lives without anyone medical ever commenting on it.
The situations that do warrant an eye exam are straightforward: heterochromia that shows up new in one eye (especially after age 20 or so), a color change accompanied by pain or vision changes, a pupil that looks smaller or larger than the other side, or heterochromia in an infant alongside hearing problems or unusual skin or hair pigmentation. Any of those combinations raises the possibility of an acquired cause or a syndromic condition like Waardenburg or Horner syndrome.
A general rule of thumb: heterochromia that has been stable since childhood and comes with no other symptoms is overwhelmingly benign. Heterochromia that is new, progressive, or paired with other findings deserves investigation.
Heterochromia in Dogs and Other Animals
Heterochromia is far more common and conspicuous in certain dog breeds than it is in humans. Siberian Huskies, Australian Shepherds, and Dalmatians are well-known for complete heterochromia, where one eye is blue and the other brown. Research involving genetic testing of over 6,000 dogs identified a specific duplication on chromosome 18 that is strongly associated with blue eyes and heterochromia in Siberian Huskies, separate from the “merle” coat-color gene on chromosome 10 that can also produce blue eyes.14PubMed Central. Direct-to-consumer DNA testing of 6,000 dogs reveals 98.6-kb duplication associated with blue eyes and heterochromia in Siberian Huskies In dogs with piebald (white-spotted) coat patterns, the same melanocyte migration story applies: white patches on the face sometimes extend to one eye, reducing melanin there and leaving it blue.
Cats show a similar pattern. White cats or those with large white patches are especially prone to having one blue and one yellow or green eye. In both cats and dogs, complete heterochromia on the blue-eyed side sometimes correlates with deafness in the ear on the same side, mirroring the neural-crest connection seen in human Waardenburg syndrome. Despite the visual drama, heterochromia in pets is usually harmless and does not affect the animal’s vision.
Cosmetic Procedures That Change Eye Color
Social media has fueled interest in artificially changing eye color, and several surgical approaches now exist, though they sit on very different points of the risk spectrum. A recent narrative review described three primary techniques: cosmetic iris implants, laser iris depigmentation, and cosmetic keratopigmentation.15PubMed Central. Surgical Techniques for Cosmetic Eye Color Change: A Narrative Review
Cosmetic iris implants are thin silicone discs placed over the natural iris. They were originally designed to treat congenital iris defects or traumatic iris damage but have been repurposed for purely aesthetic purposes at overseas clinics. The complication rates are severe, including glaucoma, progressive corneal cell loss, and permanent vision impairment. Major regulatory bodies do not approve them for cosmetic use.
Laser depigmentation uses a laser to break apart melanin granules in the front layer of the iris, gradually lightening a brown eye toward blue over several sessions. The results can look natural, but downsides include patchy pigmentation, light sensitivity, and temporary pressure spikes inside the eye. Long-term safety data remain thin because the procedure is still relatively new.
Keratopigmentation takes a different approach entirely, tattooing a thin layer of pigment into the cornea (the clear front surface of the eye) rather than altering the iris itself. It offers more color customization and has a shorter track record of complications compared to implants, though it is still an invasive procedure that carries real risks. None of these techniques are routine or universally endorsed, and anyone considering them should understand that the eye tolerates cosmetic intervention far less forgivingly than, say, the skin.
Why Some Eye Colors Seem to Shift With Lighting
People with lighter irises, particularly those in the blue-green-gray range, often notice their eye color seems to change depending on lighting conditions, clothing, or even their mood. This is not heterochromia; it is an optical phenomenon. Because lighter irises have less melanin, incoming light interacts more with the structural fibers of the iris, and the perceived color shifts depending on the wavelength of ambient light. Warm indoor lighting can make blue-green eyes look more green, while overcast daylight might push them toward gray. Pupil size also plays a role: a dilated pupil exposes less iris, which can make the visible rim look darker or more saturated.
This shifting quality is sometimes mistaken for central heterochromia, especially when someone notices a gold ring near the pupil that becomes more visible in certain light. True central heterochromia involves a genuine and consistent color difference between the inner and outer iris that does not depend on lighting angle. The ring stays the same color whether you are indoors, outdoors, or photographed with flash. If the color difference only appears under certain conditions, it is more likely a normal optical effect of having a low-melanin iris than a distinct form of heterochromia.