DO NOT attempt this yourself; especially the first time!
The darker your hair, the longer it will take to bleach the color out. And for the silvery white you have to get ALL the color out first, then add the dye. Unfortunately, the longer the bleach is in contact with your hair, the more damage there will be to the cuticle (texture). Its unavoidable, but up to a certain point it is repairable. Find a professional in the best salon you can afford and specify you want someone skilled with drastic coloring. Interview the salon and/or hairdresser in person and take a color picture with you.
After they do it the first time you will have a better idea of the process, if you have to maintain it yourself because of the expense of the professionals.
I have those two shades naturally thanks to time, but periodically I get stupid and try to bleach more of it out to the silver. I have regretted it almost every time because of the process and the upkeep; or the look if you get if you don't keep it up. My silver streak since high school is finally the major color of my roots now, but I can't wait for a bit more length to just add that metallic touch.
