You must have them in direct hot sun. Petunias prefer part shade during the hottest part of the day. Dark colors will bleach in a hot sun. Especially if you have had very hot weather. If possible move them so they are a bit more shaded, or trade places with a lighter colored one

Thanks oilpainter once again your help is appreciated.they were in direct sun during a week long heat wave.They look like they are all ready recovering.I'll plant a different color next year. For now maybe I'll stick a beach umberella next to them during extream heat.

Grandifloras have either single or double flowers. Flower blooms can be messed up with heavy rain and too much exposure to sunlight.

The dark purple seem to have less tendency to bleach out IME. Red petunias, in general, seem to bleach out easily in strong sun which is why I don't bother with them despite liking the deep red color some varieties have.