This article is from the Tattoo FAQ, by Stan Schwarz with numerous contributions by others.
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.
 
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