Heterochromia
When a pup has two different colored eyes, it's called heterochromia. This literally means two different colors. While it can occur in humans, it's not very common. It's much more common in dogs, cats and horses. The distribution of melanin in the iris determines eye color. More melanin means he'll have a darker eye, like brown. Less melanin results in green or blue eyes.
Heterochromia can also cause one eye to lack melanin altogether. When his two eyes have different amounts of melanin, they'll be two different colors. The most common combinations include brown, blue and white.