I bought the large for my pushing a 36 waist and they fit well.  I do lots of sweating during cardio workouts and these work well.  I'm not sure why the ends are gray.  I noticed when stretching my legs up at the end of an interval class that the gray will sometimes show above my black gym shorts.  For an hour or two of gym work, no fly is no problem.  They were so comfortable that I decided to wear them to work went to the men's room and well, I missed the fly :)