As far as the Gelbvieh breed goes, the following applies. (I think it applies to most other solid colored black or red breeds of cattle as well).
If the cow or bull is black (homo or hetero) then they cannot and do not carry the diluter gene, as it is dominant over the non diluter gene and would cause the black to be gray.
So... the homo black bull will throw all black calves and some could dilute to gray if any of the cows have the diluter gene and express it. Even if the cow is diluter, that does not mean that she will definately have a gray calf. If the cow is hetero diluter her offspring will express it some of the time. A homo diluter cow will throw the diluter gene every time, which means the calf regardless of color will express the gene and show a lighter version of the "base" color. A blond calf is likely diluter and a dark red is likely not, but even a dark red one can have the diluter gene. I have a 14 year old medium red Gelbvieh cow that has had black calves, red calves, and gray calves. So I know that she is a hetero diluter. Her pedigree with some known red diluter ancestry backs this up.
To add even more to the conversation, the above assumes that the black is black and the red is red, with no white markings, as the spotting gene could even cause more variation to the calf crop.
A prime example of this is the homo black Gelbvieh bull Echo. Although he is homo black, he is notorius for white spotting his offspring. To attest to this, Almost every calf I ever had from him or one of sons or daughters had white navels, or white stars on the forehead, or white switches, or white stocking feet, etc.
My 2 cents.
Smoky