Pick a color
There are four possible colors for a healthy purebred Doberman: black & tan (also called black & rust), red (also called brown), blue and fawn. A genetic mutation also exists in the white albino Doberman but it cannot be registered with most national Kennel authorities around the world. Albino Dobermans are prone to major eye, skin and other medical issues.
Although one would think picking one of the four colors should be a simple question of taste, there are also a few medical considerations to keep in mind. Blue and fawn Dobermans are also prone to skin & coat diseases and lead to possible medical complications. The blue coat is a dilution of the black, and the fawn coat is a dilution of the red. Because of the fact that the blue and fawn genes are scarcer, combined with the possible medical issues, blue and fawn Doberman are generally less common.
While some few breeders solely specialize in one single color, most will routinely have mixed litters of black & tan with red puppies. Although this is the case, some breeders often have a waiting list for certain specific combinations of sex & color. As such, absolutely wanting a specific combination may force you to step up your research, and even be willing to wait up to a year to get the healthy puppy you really want.