I use liqua-gel food coloring and an un-inked ink pad or stack of paper towels for the pad. I have a tutorial on my website here: http://www.juliausher.com/blog...d_springtime_cookies
I prefer liqua-gel to airbrush coloring and gel coloring. Airbrush coloring is relatively loose and slippery (I feel there's more potential for the stamp to move and the coloring to smudge). And gel coloring, while it definitely works, is more tacky and you have to be more careful to blot the pad so the stamp does not get over-inked.
There is some debate about the food safety of craft rubber stamps, but that is a topic for another forum.