Heterochromia means a difference in colour — either between the two eyes, or within one iris. It usually needs no treatment, and most people with it have no other eye or health problems.
Heterochromia
Each eye is a noticeably different overall colour from the other — most commonly present from birth and lifelong, occasionally acquired later.
Central heterochromia
One eye shows a distinct inner ring of colour around the pupil that differs from the rest of the iris — commonly a golden, amber or green ring within a browner or greyer outer iris.
Sectoral heterochromia
A distinct wedge or patch of different colour within one iris, rather than a full ring (central heterochromia) or a fully different second eye (complete heterochromia).
Complete heterochromia
The two irises are different overall colours from one another — for example one brown and one blue — rather than one iris containing two colours. This is the pattern most people mean by 'different coloured eyes', and it is distinct from the central and sectoral patterns, which occur within a single iris.
The three patterns look quite different from each other, and all three get confused with ordinary iris variation. These three questions separate them. They turn on one thing: whether the boundary between the two colours has an edge you could trace, or whether one colour simply fades into the other.
This is the distinction that decides between complete heterochromia and everything else. Be strict with yourself: nearly everyone's eyes differ a little, and a difference you have to hunt for is not what the word describes.
A segment with an edge to it — like a slice of a pie, or a blot — is sectoral. It has a boundary you could trace. A soft gradient across the iris is not this.
The word doing the work is ring. A defined band with an edge where it stops is central heterochromia; a warm inner area that fades outward is ordinary iris structure, and is what most hazel eyes look like.
Iris colour comes from how much melanin sits in the front layer of the iris and how the tissue behind it scatters light. Heterochromia is what you see when that quantity is not the same everywhere — between the two eyes, or across one iris. The pigment cells that populate the iris arrive during development and, in most people with heterochromia, simply did not distribute evenly. That is the whole mechanism in the ordinary case: not a malfunction, and not a marker of anything else.
Because it is a difference in pigment quantity rather than in pigment type, the two colours in a heterochromatic iris are almost always related — brown beside green, amber beside brown, blue beside grey. A genuinely unrelated pair is rare, which is one reason dramatic examples circulate so widely and are so often edited.
Heterochromia can also be acquired, which is a different matter entirely and the reason the note below insists on the distinction between something you have always had and something that changed.
More claimed heterochromia comes from cameras than from irises. A flash sits to one side of the lens, so one eye catches more of it than the other and comes back lighter. Any light source not directly in front of a face does the same thing more gently. Automatic white balance then adjusts the whole frame, which shifts both eyes in the same direction but does not equalise them. A single image is therefore weak evidence, and a single image with a flash is close to none.
The test that costs nothing: look in a mirror, in indirect daylight, with the light coming from in front rather than the side. A real difference survives that. Most photographic ones do not. More on what light and cameras do to eye colour.
Heterochromia has several possible causes, most commonly present from birth with no other effect on eye health. This page describes the visual patterns only — it does not diagnose why any specific person's eyes look the way they do, and cannot rule in or out any particular cause.
A photograph — especially one taken with flash, in mixed lighting, or processed by phone software — is not reliable evidence of heterochromia. Reflections and white-balance shifts can make a single eye look a different colour in one shot without any real difference being present.
Heterochromia present since birth is usually lifelong and doesn't need treatment. A new or unexplained change in the colour of one eye — as opposed to a difference that's always been there — is different, and can be worth mentioning to an eye-care professional as a precaution.
No advertising is shown on this page — it discusses when a change in eye appearance can warrant professional assessment, and Colourwise never places an ad beside that guidance.
Source: American Academy of Ophthalmology. This page is general information, not medical advice — see an eye-care professional for any individual concern.