How does a person have two different colored eyes?

Eye color. This may be congenital resulting from either horners syndrome or it could be a specific eye problem causing loss of pigment or atrophy of the iris of one eye. If it is aquired then it can be due to iris damage either from inflammation, trauma or surgery. The condition is called heterochromia.
Genetics. It is rare. Google it! it is very interesting. At your next eye exam, ask your eye car professional.