From memory I think Andy Wedomre did tats on his display model Sidhe Sorceror types. They were "tribal" style for the most part and fairly muted. They looked good (but then Andy did them so they would do

)
I've done a fairly bright woad blue type tats and/or war paint on a few of my gaels but so far have stayed away from diong so on my sidhe (mind you, the few of them I have painted are purple...)