The entire extent of our evidence for any sort of paint decoration comes from Ahmad Ibn Fadlan's Risala (an account of his diplomatic mission to the Bulgars, and his travels through the Rus'). In his account he says "Each man, from the tip of his toes to his neck, is covered in dark-green lines, pictures and such like." (translation by James Montgomery). This has been interpreted as evidence of tattooing or some kind of body paint. In either case, while this is an eyewitness account, it is not one that is corroborated by any other source or archaeological finds. This, at its most generous, indicates that one group that we would now identify as the Rus' along the Volga river had extensive body paint traditions, and at worst is an invention to contrast the "barbaric" Rus' with his elite Arabic audience. More work needs to be done on Ibn Fadlan's cultural context, but that work is ongoing.

Certainly, dyes of various kinds were available for textiles, including woad, as you mentioned, and it is not impossible that these were used as war paint in addition to being used for clothing and luxury goods; however, there is nothing I am aware of in either Continental chronicles or later sagas to suggest that war paint of any kind was commonly used among the Vikings.