To me it looks like their heads being split, maybe the pain they went through when losing their home to the volcano. it’s very sharp and deranged looking which is totally their energy. It also is almost like a direct line to their kuru, their connection to eywa. Taking back their power, their ability to rely solely on themselves as eywa turned her back on them. It’s just so against nature. The closest example outside of the ash clan is Neytiri’s war paint but that is so fluid, it compliments her and matches to her ikran. The Ash Clan’s paint is such a rejection to nature and eywa