Good question. I would like to be able to back up my decision to paint tattoos with some obscure academic reference, but truth is I thought this guy looks a bit plain with his top off, he needs tatts!
I generally go for whatever I think looks good rather than what history dictates. And there's a good chance that my choices are as accurate as any historian's interpretation of the scant/non-existent "data" on the vikings. With the clothing I reckon the re-enactors give us as good a window into the real world as anything else, some of them go to huge lengths to use ancient dyes and methods, though again this is as much guess work as anything. I am currently painting shields and there is a real dearth of information. One Norwegian(?) shield had traces of blue paint, and in one (saga?) reference shields were painted black and yellow, while in another red was used to signify aggression. I have only read second hand accounts of these "facts" so could be re-circulating urban myth for all I know. I just go with what looks good to me but that is not anachronistic, in my mind at any rate. As I say, it's all pretty much guess work when it comes to vikings.