if you are breeding for specifics then it is best to start with known genetics, not pot luck genetics from a mill bred source of rats (i.e. petco/petsmart, etc) or from pet shops that buy from local hobby breeders you are not able to directly speak with (& even if you do speak with them, they may not be able to tell you anything about the genetic history)

You will need to know several generations of genetic history in order to predict accurately the color & coat type that you will produce