Plain old acrylic paint works very well. I've used cheap craft paint and the nice artists paints and both work well.
Just my .02¢ worth… Rub-n-Buff is a "Wax Metallic Finish"…says it right on the tube. If you happen to be using Alumilite, however, it's urethane based and will not bond with acrylic. There is something funky going on within the "molecular level" (I'm not a chemist so I'm just taking a WAG). Sooooo….If you apply a couple coats of a good high quality polyurethane over the acrylic you will avoid the bonding issues that Alumilite presents. I love the stuff. It's like chucking up a frozen stick of butter, spinning it at 3000 rpm and getting those coveted non-stop ribbons creaming off the blank. I'm guessing those characteristics are the reason for the price differences. Very nice stuff to turn!…
….By the way, Rub-n-Buff is being phased out by American Art Clay, the manufacturer of the stuff, so it would behoove you to stalk up on it if you're using a lot of it! I've been buying a small tube at least once a week from Hobby Lobby with their 40% weekly coupon for the past couple of months when I found out.