Krylon Indoor/Outdoor Paint works well and, as Clay said, they have a new paint. The new one is supposed to be made for plastics like PVC. It's called Fusion:

http://www.krylon.com/products/

I think the best end cap (at least for one end) for the PVC tube is a plastic test cap. They are cheap, and add nothing to the diameter. They take up only about 1/2 inch to the length inside. You can get them anywhere PVC pipe is sold and they are called "PVC Pipe Test Caps."