In anatomy, heterochromia refers to a difference in coloration, usually of the iris but also of hair or skin.
Eye color, specifically the color of the irises, is determined primarily by the concentration and distribution of melanin.] The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, usually, an excess of melanin indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia.
Now... that doesn't sound very sexy... but you know what.. it IS!