Heterochromia iridum

Heterochromia iridum is the scientific term for an individual having eyes of 2 different colors, determined primarily by the concentration and distribution of melanin (more = brown, less = blue). The rare condition can be genetic or acquired (due to injury, inflammation, medication, or tumors), and can affect one (in which it is called central heterochromia or heterochromia iridus) or both eyes. It occurs in animals (such as cats and dogs), which almost always have one blue eye, and humans (examples here, here, and here).

David Bowie (shown decades ago in 1st image, and more recently in 2nd image) does not in fact have heterochromia iridum. Bowie's right eye is blue, while his left eye appears brown or green, depending on the light. But this is because he has a permanently dilated pupil, which affects his depth perception. The injury was received at school in 1962 when a friend punched him in the eye while wearing a ring. Doctors saved his sight, but the efforts required a 4-month hospitalization.