I want to call my girlfriends hot pink hair tacky
Why do you think that hot pink is tacky?
I hate the color of her hair.
It is ok to not like something about your girlfriend. You should be honest with her and if you don't like it you might need to break up.
You're not helping, I'll still like her.
Well everyone is entitled to their own opinion. You can tell her kindly that you like another color better. But you can't try to force her to change it.