What about fabric dyes? I think most white PUL is done with a white polyester fabric, and I don't know which dyes would work best, but I'm sure there's something out there for dying polyester. Anyone know about this? (I've been wondering as well about dressing up white PUL)
As for acrylic paints, you can pretty much get them at any craft type store, and if you have a Michaels or AC Moore craft stores near you, they are always having paints on sale.
Have fun!