Why do the akatsuki paint their nails? They even do it in matching sets for their teams. It's kinda cute. I can imagine them giving each other manicures and really scrutinizing the color. "But Itachiiiii, it's too hard to pain your family crest lets just be purple." - itachi sighs and begrudgingly removes the perfectly painted uchiha symbols-
I honestly can’t think of much of a reason beyond aesthetic? There are a couple of things one could do regarding nail polish and fighting but I don’t think that any of the Akatsuki have any practical applications for theirs.
And I like the way you think. I imagine Hidan being unusually anal about it, like, “Dammit Kakuzu, you smudged it. I could be killing people but no, now I have to fix this.”
Except with more cursing.