Aries: Pinkamena. Purple. You were sad and she wanted to lighten up your mood.
Taurus: Laughing Jack. Literally gOlD. He was bored.
Gemini: Nina the Killer. Purple with pinkish streaks. She got mad at you for sleeping in for too long.
Cancer: BEN Drowned dyed your hair pink because it was funny to him.
Leo: Jeff the Killer. Grey. It was a bet.
Virgo: Slenderman. He was jealous of your hair and so he decided to destroy it by dyeing one side purple and the other pink.
Libra: Jason the Toymaker. He shaved your hair off while you were asleep and you woke up wearing a white wig.
Scorpio: Kagekao. Very light brown, you wanted him to do it. He was proud of it and then he spilled wine all over it (by accident, of course).
Sagittarius: Masky. You told him you had a dream about dyeing your hair and so he decided to help you have your dream come true by dyeing it black and green.
Capricorn: Sonic exe. BRIGHT-BrIgHt orange. You lost in his game.
Aquarius: The Doll Maker. You almost cut his head off when you sleepwalked and he dyed your hair to dark green for revenge.
Pisces: remember how Laughing Jack tried to get rid of you? You dyed your hair blue and got a side shave so that you don't get recognized easily and came back.