Woad, derived from the plant Isatis Tinctoria which is a fairly non-descript yellow flowered plant( seeds are obtainable in the UK from Many seed merchants) is an incredibly effective tissue dye,( I KNOW this from experiment) and if you get it on Body parts it will persist until it wears off and the skin replaces itself, this can be up to 2 WEEKS , If you are really unlucky, and you paint patterns in Woad on your person and it is Sunny, the woad selectively absorbs UV and after the blue has gone you can have the patterns in bright red SunBurn ( tan, if you Tan) on your person, visible for MUCH longer.
Also the preparation of the Woad dye can Make you VERY unpopular with the neighbours as it involves boiling the plant in urine( altho' dilute Ammonia works as well,it hardly improves the effect) the effect on your neighbours can be easily predicted.