Just thought I'd share my experience..
A couple years ago I decided to try Jolen.. I put it on my arms (worked like a charm) and my upper lip....!!!! It made this dark gray shadow on my lip EXACTLY in the shape I applied it. OMG. To say I was scared is an understatement. I started crying and didn't go down for dinner.. I thought it wouldn't go away for a long while. Luckily, after I cried myself to sleep and woke up: it was goooone! Good thing too, because I would have never went outside like that! Forget school and eating! I would have been some decaying, under-educated, malnourished girl... WITH AN ODDLY SHAPED GRAY PIGMENTATION SHADOW IN THE CENTRE OF MY FACE!
Now it's a funny story,but really... Be careful. Do it on a Friday evening so there are almost 3 days to re-coop.
So who knows, everyone's body is different and reacts differently. I would say the arm is fine though.
AND. You can use hydrogen peroxide to gradually lighten hair if your skin is darker and you want a more subtle effect.