The backdrops are .060 sheet styrene painted with two shades of blue flat latex paint from Home Depot that I blend together. I use a typical sky blue on the upper two-thirds. I no longer know which Glidden color I used originally, but Glidden's "Wild Blue Yonder" is close, though a shade lighter. On the bottom third, I use a very light blue (almost white) from Glidden called "Ice Age." I apply both to 4 or 5 foot section of backdrop with rollers and then I blend them together with a wide brush while they are still wet to create that hazy transition.
The Glidden website is great for comparing shades.