I used airbrush ink, because it's transparent and waterproof. I wanted the grain of the rawhide to show through, but didn't want to risk smearing if it got wet. (Because nobody ever
forgets to take their drum inside after a night of drumming around the fire.)
Liquid acrylic would have been a more logical choice, but I didn't have the right colour. I'm guessing that leather dye would have been okay too but I'd want to do a little test to make sure it didn't run when wet. The great thing is that the rawhide is transparent enough (even though this is quite thick) that you can put a pattern underneath it and trace onto the surface.