I use Ceramcoat to paint the inside of the tube. I use long pipe cleaners to paint the inside. Make sure to "defuzz" them before use, however, or you'll end up with fibers in the paint. You can remove the loose fuzz by running them through your hand or a cloth a number of times.
I use epoxy to glue in my tubes, and I tint that with Testor's. It doesn't take much enamel paint to make the epoxy opaque, and I find that the epoxy cures almost as fast as non-tinted epoxy. This is a great way of hiding any potential air bubble or voids in the adhesive that might otherwise be visible with a translucent blank.
I no longer paint the tubes as that just adds too much thickness for my liking. Besides, the painted blank AND tinted epoxy has never left me with with any bare brass showing ever.
I will, on occasion, use a colored Sharpie to tint the brass tube. It adds nothing to the thickness of the tube, while it adds another layer of "protection" from the dreaded brass shiny spots. YMMV