Heterochromia iridum is a difference in coloration of the iris and is a results of the relative excess or lack of melanin (a pigment). According to Wikipedia:

It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury… 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).