like with all breeds-- it doesn't matter if two blacks are mated together-- unless one is homo black--- a chance of red calves will pop up-- 25% actually.
Since you don't the background of the cows- but the Gelbvieh breed is known for originally being a red breed-- I would assume that many of these cows are red carriers. My guess is that many of these cattle are carrying black from the Angus, and red from the Gelbvieh-- even if their sires might be black Gelbvieh bulls. Or, maybe the Angus in them is red?? Things could get real interesting if someone was breeding red carrier Angus cattle to Red Carrier Gelbvieh cattle.
Since I don't know anything about the Gelbvieh bull you mentioned breeding to-- I can't tell you if he's hetero, or homo for black. Obviously, since you got red calves from him-- he's not homo.
Now the red tinge, could be something completely different. Time and time off someone on here, leanbeef, and I have had many discussions about coat color in cattle-- and we've come to the conclusion that the wildtype color pattern runs rampant through many breeds-- and I'm almost certain that the tinge that appears in many breeds is a result of such genes. If I remember correctly, the black around the eyes is also characteristic of that trait-- and for sure is of the Gelbvieh breed.
To answer your question-- yes , in order to ensure that all calves will be black-- you have to breed them to a homo black bull, or have a homo black dam-- unless you are going to be playing around with certain breeds-- where even if black-- they can have other color traits show up-- such as the roaning in the shorthorns, the dilution in the Char, and the spotting of the Sim.
