I think you made the right choice buying the red cows considering the price spread. The red cows, assuming they do have a high percentage of Red Angus blood, may also be able to tolerate the hot, humid summer days a little better than black cows.
In the future you may want to consider using a homozygous black Gelbvieh or Simmental bull on those cows. You'll still get black calves, plus you'll get calves that grow quicker, produce a higher yielding carcass, and some wonderful replacement heifers that should make fine mommas.
If you're not ready to switch to a purebred Gelbvieh or Simmental bull, you may want to consider a half Angus/half Gelbvieh or half Angus/half Simmental hybrid bull. You'll still get some of the hybrid vigor and leaner carcass, but you won't sacrifice as much marbling.