Headcanon where old wizarding families have distinctive hair for magical reasons, and that witches and wizards marrying into the family find their own hair slowly transforming to fit their new family name. It would explain why ALL Weasely's have flaming red hair, why Narcissa is a platinum blonde like her husband despite her sisters being described as having darker hair. It would also explain why Tonks favored mousy brown hair like Remus' while she was trying to convince him to elope.
Hermione's hair slowly turning more auburn after she marries Ron
Fleur's veela heritage combating some of the magic leaving her a gorgeous strawberry blonde
Pictures of Lily that show her hair getting steadily curlier over time.
Draco watching Harry for signs of his hair color lightening for months before waking up and screeching in horror at the soft waves developing in his hair.