Glastum, Woad and Isatis tinctoria are all the same plant in the end.

Woad got the common name of glastum from the writers that made the mistake of thinking the woad plant that makes good cloth dye also made the body paint or tattoos. They gave woad the name the romans gave to the colour of celtic body paint/tattoos and blue green glass in more recent times.

Glastrum of the romans, and glas of the celts mean blue/green, not the true blue colour you get with woad. Woad is a violet blue if you look here you can see the colours: http://www.boutique.bleu-de-lectoure.com/catalog/popup_image.php?pID=29. Copper and iron both make good blue green pigments though. You can change the colour in various ways, but the chap on that first site seems to have tried it all and found it rubbish for body paint.

I've thought the same myself, about the iron for tattoos. I'm sure one of the ancient sources says the picts tattooed themselves with hot iron, which some people took to mean they branded themselves, but which I took to mean something to do with the iron pigment tattoo process.

Woad is an astringent, so it might have been used in preparation.

Woad and bear fat makes a silvery body paint which confuses hunting and hunted animals alike according to one author, and was used by hunters. Got me thinking... Glas for the celts also has a another meaning of grey or clear. So maybe it meant invisible, to those hunters, masking their scent and hiding them.

Saw this about woad as a cancer preventative...
http://news.bbc.co.uk/1/hi/health/4783831.stm

All gets you thinking huh? Or do I just wander off topic annoyingly?