Concerned that you'll end up with a greenish tattoo with little bits of red or yellow? Worry no more! Today's inks run the entire gamut--and it would not be terribly sarcastic to take a Pantone color chart with you! Most tattoo inks are metal salt-based pigments that are not made specifically to be used under the skin, and have not been approved by the FDA for this purpose. The idea is that for most people, these pigments are inert and cause no problems. Some people have been known to have allergic reactions; any reputable artist should be willing to provide you with a small "patch test" of the colors you desire. This is required in the state of Arkansas.
Tattooist Uncle Bud Yates (Pikes Peak Tattooing) says some artists use acrylic-based pigments, which he feels may be more troublesome than the metal-based pigments for some with sensitive skin. Best to ask your artist first.