It happens occasionally: a cat has eyes of different colors. The most common combination is one blue and one amber, and you’ll usually find it among white cats.
It’s a genetic oddity that is little understood. Some people find it unpleasant to look at on first glance, but owners of these “bi-eyed” cats claim that the distinctive look does grow on you after a while. (And considering that these cats are usually white, it does add an extra dash of color.)