There are a lot more possible options than just those 2. The only thing we can tell for sure is that Alleria uses war paint/tattoos at least since she joined the fight alongside the Alliance, and that at least some part of the Elven Rangers following her during the WC2 campaign did as well. Since no further info has been given about it, anything from it being a common thalassian cultural expression to it being used only among the farstriders to it being just a niche thing Alleria herself came up with and some of her followers adopted with it not at all being part of thalassian culture/heritage beyond that, or even other options aligning to the few hard facts we have is a possibility (or Blizzard could retcon things as always, but obviously things sticking to the current facts is preferable).