You can avoid a forever tattoo by using short-term, henna-based ink painted on the skin. Be careful, though. As this picture shows, even these tattoos can cause allergic reactions. Red-brown vegetable henna is approved by the FDA only for hair color, not for skin designs.
Stay away from "black henna" or "blue henna" tattoos. The color may come from coal tar, which often causes severe allergic reactions.