Honestly, people just... do dye their hair, I personallly change the color and the cut once every few years, so I don't see why the character can't have that. If they don't seem like a person who is particularly into changing themselves randomly, you can introduce some small twist on why they are doing it (falling in love, breaking up, going through some emotional moment, totally just digging some model with this hairstyle). That is, if you don't have a magical world and the character can just doing by getting magical abilities or drinking an unknown potion. I think if you have a good explanation it can be fun, for readers too!