There are only two or three people in this country who breed for Isabella's.
They are generally not popular because their coats are so poor - fine when a pup but by the time they reach two years old they generally have only an undercoat if anything at all. You are unlikely to do well in the showring with a fawn because of the coat quality unless you are very, very lucky.
The blacks have the best coats although in recent years the browns have been equalling them.
Blues and fawns are not really favoured or bred for because of their coat problems. To give you some idea of numbers last year there were
2562 blacks registered
556 browns
33 blues (some of these were a complete litter produced from two blacks and must have been a shock to the breeders as there were no blue ancestors for at least 4 generations)
12 fawns
If you are interested in the colour combinations and what produces what visit
Dobermann Colour Chart .
To answer your question, yes you can mate a fawn to a black (it is of course best if you have carried out hip,
eye and
vWD tests first and choose any potential parents accordingly :-))
Christine