I had a bunch of epoxy from Anchor Bond, and they had some coloring paste available (white, yellow, red, blue, black). It got mixed into the resin, then the hardener was added. It seemed to work pretty well. Color and fuel proofing in one step.

As far as I can tell, Anchor Bond epoxy is no longer available, and I am out of resin and hardener. I still have most of the colored pastes. I'll probably try mixing it with another brand to see if it works.