I always imagen them as savage picts or scots (red/blonde hair), if you paint a giant put them freakls

. Myabe some lines blue and white over the skin in some figures could simulate tatoo, but in my humble opinion I would avoid tatoo all the figures in the unit.
I seem them as Slain comicbook as a source of inspiration.
Also I am not GW euridit so... maybe my vision is not what the company has