Can you just add food coloring to any recipe? if so ...when?

make a Pale ale, pilsner, IPA or something light in color. (SRM lighter than 9) then add the food coloring in the bottling bucket just before bottling....taaadaa
and be prepared to buy a new bottling bucket unless you want it to be green...and other beers to possibly be green.
I assume you are making something for St. Pattys? Just put the food coloring next to the kegorator and let people add a few drops to their pint when they pull it. That's how it's done at the Irish pub I go to for St. Patty's.