Having two different colored eyes, a trait called heterochromia, happens when one eye (or part of one eye) ends up with a different amount of melanin pigment than the other. It affects roughly 6 in 10,000 people. Most of the time it’s harmless and present from birth, but it can also develop later in life from eye injuries, inflammation, or certain medications.
How Iris Color Works
Your iris gets its color from pigment-producing cells called melanocytes that sit in the front layer of the iris. What matters isn’t really the color of these cells, but how many pigment granules they contain and what type of pigment those granules hold. There are two main types: eumelanin, which ranges from brown to black, and pheomelanin, which ranges from red to yellow. The relative mix of these two pigments, combined with how densely the pigment granules are packed, determines whether an eye appears brown, green, hazel, or blue.
Brown eyes have a high concentration of eumelanin in the front of the iris. Blue eyes aren’t actually blue pigmented. They have very little melanin, and the color you see is light scattering off the iris structure, similar to how the sky appears blue. Green and hazel eyes fall somewhere in between, with moderate melanin plus some pheomelanin contributing warmer tones.
Three Patterns of Heterochromia
Not all heterochromia looks the same. It shows up in three distinct patterns:
- Complete heterochromia: Each eye is an entirely different color, like one brown eye and one blue eye. This is the most recognizable form.
- Sectoral heterochromia: One iris has a wedge-shaped section of a different color, like a pie slice. The rest of the eye is one uniform color, but that one segment is clearly distinct.
- Central heterochromia: One iris has an inner ring around the pupil that’s a different color from the outer iris. It often looks like colored spikes radiating outward from the pupil.
Central heterochromia is the most common of the three and is frequently mistaken for hazel eyes. The key difference is that central heterochromia creates two clearly distinct color zones, while hazel eyes blend colors more evenly throughout the iris.
Born With It: Congenital Causes
Most people with heterochromia have had it since birth or early childhood. During fetal development, melanocytes migrate into the iris and begin producing pigment. If something causes the melanocytes in one eye (or one section of an eye) to produce more or less pigment than those in the other, the result is two different colors. In many cases, this is simply a random variation with no underlying medical cause. It runs in some families, though no single gene has been pinpointed as “the heterochromia gene.” Eye color itself involves multiple genes working together, and small variations in how those genes are expressed can produce asymmetric results.
In rarer cases, congenital heterochromia signals a genetic condition. Waardenburg syndrome is the most well-known example. It’s a group of genetic conditions that affect pigmentation throughout the body, causing very pale blue eyes, two different colored eyes, a white forelock of hair, or patches of lighter skin. It can also cause moderate to profound hearing loss in one or both ears. Some forms of Waardenburg syndrome affect the intestines as well. Another condition, Horner syndrome, can cause one eye to appear lighter if the nerve supply to that eye’s iris is disrupted, reducing pigment production on the affected side.
Developing It Later: Acquired Causes
When someone’s eye color changes in adulthood, the list of possible causes is different and generally worth investigating. Eye injuries can damage melanocytes in one iris, causing it to lighten or darken over time. Chronic inflammation inside the eye, called uveitis, can alter iris pigmentation gradually. One specific form, Fuchs uveitis syndrome, is strongly linked to heterochromia. It’s a low-grade, long-lasting inflammation that slowly changes the color of the affected eye, typically making it lighter. Most people with Fuchs uveitis syndrome eventually develop cataracts, and up to 59% develop glaucoma, but the overall prognosis is good when those complications are treated. About 85% of patients who need cataract surgery achieve good visual outcomes, and only about 10% of affected eyes show worsening vision over time.
Glaucoma eye drops are another surprisingly common cause. Prostaglandin-based drops, which are among the most widely prescribed glaucoma medications, can gradually darken the iris of the treated eye. These drugs stimulate the pigment-producing cells in the iris to ramp up melanin production. If you’re using drops in only one eye, or if one eye responds more than the other, the result can be noticeably mismatched eye colors over months or years. Eyelash growth serums use the same class of compounds, and some users have reported iris darkening as a side effect.
Growths on the iris can also create the appearance of heterochromia. Iris nevi, essentially freckles on the iris, appear as flat, darkly pigmented spots. They’re usually harmless. In people with neurofibromatosis type 1, small yellowish-brown dome-shaped bumps called Lisch nodules develop on the iris surface. These appear in 90% to 100% of adults with the condition and can make the iris look speckled or multicolored. Very rarely, iris color changes can signal a tumor, either originating in the eye or metastasized from elsewhere.
When a Color Difference Is Just Cosmetic
If you’ve had two different colored eyes for as long as you can remember and have no other symptoms, it’s almost certainly benign. A large study that examined over 25,000 people found that the prevalence of heterochromia without a pathological cause was about 0.063%, confirming that most cases are simply cosmetic quirks of pigment distribution. These cases don’t affect vision and don’t require treatment.
Children born with heterochromia are typically evaluated to rule out conditions like Waardenburg syndrome, especially if there are other signs like hearing differences, unusually pale eyes, or pigmentation changes in the hair or skin. If nothing else is found, no further monitoring is usually needed.
When Eye Color Changes Later
A new difference in eye color that develops in adulthood is a different situation. If one of your eyes is changing color and you’re not using any eye drops that could explain it, that warrants an eye exam. The color shift itself isn’t dangerous, but it can be an early visible sign of inflammation, pressure changes, or other conditions inside the eye that benefit from early treatment. This is especially true if the color change comes with blurred vision, eye pain, light sensitivity, or visible floaters.
Even without symptoms, a gradual lightening of one iris compared to the other can indicate low-grade inflammation that’s easy to miss. Fuchs uveitis syndrome, for example, is often painless and develops so slowly that people don’t notice the color change until someone else points it out or an old photo reveals the difference.