I don't want to alarm you or worry you but a couple of years ago when I was rescuing ferals, I thought that one of them had one blue eye and one brown eye. When I was finally able to trap her to take her in to be spayed, the vet looked at it and immediately said "This is a tumor and the surgery can be expensive." (It turned out she had other things wrong w/her, too, so I had to make the decision to have her put to sleep.) So just to be safe, have your vet check it out before it gets out of hand. Good luck.