I believe if you put up a white roof and then paint it red it will fade, but if you decide on red in the design then the metal will come red from the factory and it wont fade. I also don't think there is a major difference in price between the two metals but like someone already said, the darker roof will produce more heat. If there are fans and vents in the building like there will be if there is no air conditioning than alot of that heat shouldn't reach the field level.
As far as the turf goes. I will gladly give the first hundred dollars and some time to make sure the turf is painted over, not cause I don't like the Saints either.