Only 11 out of every 1,000 Americans have heterochromia, a condition that results in two differently colored eyes. This trait is commonly found in animals and is relatively rare in humans. One eye can be blue, while the other hazel. Or one eye may be brown while the other is blue. The condition can be hereditary and harmless or it can be caused by a disease or syndrome like neurofibromatosis, which is when tumors form on nerve tissue within the nervous system. An eye injury, serious infection or inflammation may also cause heterochromia.