O.k, so heres the thing....
" Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem."
It doesnt mention woad. Woad is useless as a body paint, and it doesnt stain or stay on the skin. (u can mix it with talc, but the result is a dry flaky mess). And you cant tattoo it into your skin, as it burns and causes severe keloiding of the tattoo. And again, leaves no colour.
Woad IS used as a dye (well, it was, until it was superseded by indigo, now only people who specifically WANT woad, such as reenactors - like myself - and people who do traditional dyeing for a living) however - for MATERIAL. Not skin.
So, did the picts or celts use woad? no, no they didnt... they more than likely DID paint or tattoo themselves.. but they wouldnt have used the woad plant.
Editor's note: this is a matter of constant debate amongst woadologists. If woad was used there must have been something special done to prepare it properly, and we have no idea what this might have been. -- Tem42