You can get a hetero black calf in 1 generation if using 1 black and 1 red animal. And yes, the 2nd generation can easily be homo black. My comment above about "taking 3 generations" was referring to making a black PUREBRED Simmental from a commercial or reg other breed/color cow.
If you are talking modern Simmental genetics, a huge number of PB Simmental are Homo Black.
@simme. You are correct, if the Simmentals being used were from the French or Switzerland strains.
@Warren Allison. You are also correct if they were using Simmental strains from Germany or Australia. These did not carry the diluter gene.