I think my girlfriend would be less attractive with a different color hair
How do you mean?
I think red and blond hair is more attractive than black hair
You're lucky she has black hair. You should remind her how pretty she looks. Is she thinking about changing her hair color?
She just wants to change her hair because she is really fucking ugly with black hair.
It is ok to like what you like, but you should respect her choice to change it if she wants. You might have your preferences, but you need to remember you don't own her body.