How about a "guilt trip"? "OK ... do it if you must. It's your body after all, and although your mother and I have sweated blood to try and protect you for this long, and if you want to ruin it forever by injecting coloured dye into the skin then it's I guess that ultimately it's up to you " (said with sad face and the demeanour of a broken soul, preferably as a little tear runs down your cheek!).
Might work