I never understood the Celestia / pink hair thing, is it canon?
No it's not.
The pink hair is a nod to Luna's "normal" mane after turning back from being Nightmare Moon, so the fandom said that this is pre-princess mane or something.
I'm sleepzy and tired and I haven't had pepsi and nobody's feeding me and I'm lonely and anxious for Wednesday and I can't think of more things to complain about! I hafz every right to be in a sassy mood