I 2nd this Joe acrylic water based paint works very well I have used this a lot even when casting and doesn’t react with the PR resin (please don’t comment on the blank)
the tube has been airbrushed with a blue acrylic paint - 3 layers and dried instantly with a hot hairdryer on each application, the reason for airbrushing is that I get a very even flat finish as you can see in the image that the resin magnifies any faults or imperfections on the tube, now if your only painting the tubes because of transparency reasons then a good art brush will do, you can always heat again with a hair drier to speed up the process, people ask if CA glue reacts with the acrylic paint and I so no it doesn’t but what you do have to remember is that if you are painting tubes and inside pen blanks then this will alter the size of the tubes so a 10mm hole might have to be 10.5 to allowing for the paint on the tubes (depending how much paint is used).
This is a link to the paints I use - you can buy these at any good art shop - don't buy cheaper brands... they don't work stick with daler and rowney.
Daler-Rowney System 3 Acrylics
This is just an image to show the painted tubes
Another image is my hand painted tubes with the same paints to create British camouflage
In the past I have tried many different paints, spays, furniture sprays, umbro paints and what I found is acrylic water base paints work very well - it's a quick application because you can heat dry them to evaporated the water and that in turn harden the paint - hopefully I have made it all clear.
If you’re thinking of painting the tubes for clear casting then I strongly recommend air-brushing - there are some great colours in the system3 range even some metallic paints.
Merv