Subtle changes in coat color as the dog ages is normal. If it’s drastic though you may want to play it safe and see a vet. It certainly wouldn’t hurt. But yes, some subtle changes are normal.

If by black and tan you mean the brown spots are lighter colored than the rust color, then yeah that’s pretty normal. There are variations in the shade of their markings. The markings on American Dobermans are usually lighter and Europeans are usually darker, but there are exceptions of course.

All these dogs are still the same breed. They’re all Dobermans. If you are referring to American or European Dobermans, generally the European tends to be a bit larger with more muscle mass.