Well, if they were a TRUE bay and red (chestnut/sorrel) pair, then the only possible colors would be bay or red. I'm guessing the parents were misclassified if the foal was some far-out color. It's possible that the foal appeared black at birth but will later lighten to bay or brown. Now, if the chestnut was actually a red dun, that opens up the possibilities to dun or red dun. Pictures of parents?