Have you seen designer sportswear? I don't mean Stella Mc Cartney does Adidas (although some of that stuff is lush!) I mean the Prada does a sportswear line- check on Vogue!
Obviously never meant to set foot in a gym (if you have that type of money you could pay people to get fit/lose weight/look better for you!) but there is the possibility to design better stuff, for unknown reasons mass market producers for Nike and etc just don't bother and play it safe, black black black and black.
I do think though that the reason for the brighter colours is to draw attention away from the fact that one is wearing skin tight lycra (and maybe a tad too tight being that your body swells once you get very warm!) and sweating to the speed of light!