I am not sure if this is incomplete or a bug, but when the Axolotls are in a bucket they all look pink no matter what color they are when you place them in water. Although they do revert back to their original color.

The Axolotls when picked up in a bucket all look pink when in the inventory. They do turn back to their original color when placed in the world. I am not sure wether this is a bug or an incomplete feature.