I want to be supportive of my girlfriend but I absolutely hate the idea of her dyeing her hair
What don't you like about her dying her hair?
I just don't like it when women dye their hair.
You should try to tell her the truth about how you feel, but also respect her decision to choose what to do with her own body. Can you explain further why you don't like it?
I think women look ugly when they dye their hair. It looks so artificial.
If someone wants to change their appearance they should be allowed to. I think a lot of women might be offended if you called them ugly or implied so.