She's bought a Colourista hair dye today for darker hair apparently. I've warned her that once it's done it's done etc but typically she knows best. Her hair is fairly dark brown. She's used semi permanent dyes before and even they have stuck to her hair in the past. Plus school in Sep wont allow it if its obviously bright. If I forbid it I'll be even more in bad books than I normally am. Aibu to say no?