Think color varieties not breeds.Black is the largest color family in the breed.In addition to BBS you have lavender which is black first,white can be based on black.Brown red could work if you use black over brown red hens.Red being sex linked you get clean females to use in breeding blacks with no auto red to leak in the shoulders.In theory buff could work if you use black over buff hens.I have not tried the buff cross.I assume you are trying to restore vigor.On the wheaten family buff has always been the go to color for crossing.At this point in the breed staying in the breed is best.Yes I do outcross for projects to bring in color only.What breeds are generally out crossed to b/b/s?