Scalp hair is often used as a donor source for beard hair. If you would use black hair as a filler, it would bring the black hair into the beard. Sooner or later, these black (or brown) hairs may turn gray. There is no guarantee that your re-transplanted scalp hair in the beard location wouldn’t turn gray again sooner or later. I believe that your question is academic because no doctor I know, would transplant into a full beard with scalp hair just for color effects.