How an iris gets its color
The iris is the ring of tissue around the pupil. Its color depends largely on how much melanin its cells contain and how that pigment is distributed. More melanin generally makes an iris look brown; less allows more light to scatter through the tissue, making it appear blue or gray. Green and hazel eyes reflect a mix of pigment and light-scattering effects.
When the two eyes develop differently
Heterochromia means that the irises differ in color, either from each other or within one iris. In many people, the difference is present from birth: pigment develops unevenly between the eyes, leaving one iris lighter or darker. A single iris can also have a distinct patch or ring of another color.
A new color change is different from a lifelong mismatch. Injury, inflammation, or certain medicines can affect pigment in one eye, so a change that develops later is worth discussing with an eye-care professional—especially if it comes with pain, redness, or blurred vision. The color difference itself is often harmless, but its timing and accompanying symptoms matter.
A visible difference, not a diagnosis
Because the iris is so small, even a modest difference in pigment can make the eyes look strikingly unlike each other. That familiar “two different colored eyes” appearance is only one form of heterochromia; the same underlying variation can show up as a sharply defined patch within an iris.
