So I was just watching Descendants and I have this theory. What if Mal's hair responds to her mood like her dads. Like Hades hair is blue if he's happy, red if he's angry. In descendants one she has no real strong emotion going on and her hair is just a neutral purple. In descendants 2 it's a red-purple because she angry and angsty, in descendants 3 she was happy and her hair is blue-purple.