I use Krylon spray enamel. I also have the advantage of having 2 lathes. I use my old lathe for painting the tubes, just put them between the centers and spin them on slow while giving them a few shots of paint.
With that said, i also put them in my toaster oven on 150 degrees for 30 mins or longer to bake the paint on. No trouble with the paint coming off while inserting them into the blank. I use Gorilla glue 5 min epoxy.
I also at times paint the hole of the blank depending on how translucent the blank is along with painting the tube, if not you will take a chance on seeing the glue between the tube and blank (i paint the tube just in case a spot of paint comes off inside the hole while inserting the tube) I also use a q-tip for the hole.
And the way i see it, if your going to paint the hole in the blank, it needs to dry so why not go ahead and paint the tube also. One is not going to dry faster than the other.
With the darker colors, purple and black for example, i use white
For the lighter colors, whites, pinks, blues and so on, i try to match the color with the blank.
This works for me cause i'm no rush to make a pen from start to finish in 15 minutes...
Harry