You see lots of white pvc (schedule 40, schedule 20, or the tan cpvc) because it's cheap. Schedule 80 pvc is dark grey, harder to find, and more expensive. If you see black, that's abs. As far as I know, there is no difference chemically, once the glue is cured, on any of these products, (as far as aquarium use goes) Like keith burn said, it's just the look.