If Hux had purple hair, I would have color corrected his hair too. And for the same reasons as changing Holdo’s to a more natural color. Neither are aliens so purple wouldn’t be a natural color. Something about spending their time on hair in the middle of Armageddon seems weird. It would be like Kyle Reese taking the time to dye his hair in the middle of The Terminator while he’s running for his life and living in sewers, eating rats. Something about it just doesn’t seem right. Granted, I had to watch The Last Jedi about 40 times before it started to bother me