PLATINUM BLONDE HAIR?!?!?!?!?

Your hair will have to be bleached to a pale yellow in order to provide a good base for platinum toning. Bleach leads to dry gross hair, no way around it! First off, I'd see a colorist for such a big change, just to insure the health of your hair and your desired result. You must use the very best PROFESSIONAL shampoo and conditioner that you can afford. I don't mean drugstore crap and I don't mean knock-offs from Sally Beauty Supply