The only difference between a red and black purebred Gelbvieh is the color of their hair. I am not aware of any black fullblood Gelbvieh. You are correct in the assumption that they ascended from a black base, such as Angus, at some point in their life.
As for what color most breeders are producing, I believe that is dependent upon the market in your area. Most breeders are going to produce what sells best in their area. There are many good black bulls available, and black calves can be produced in one cross, if bred properly. However, there are many good red bulls available for those who chose to produce for the red market.