I had hair with 3 different colours in it that I eventually bleached and wanted to tone. I have used this toner before and had a good experience with it so I decided to use it again. People use toners to "tone" down the brassy or vibrant yellow colour that bleaching your hair causes.
If you have light (white) parts in your hair they might turn a little purple but will wash out. Don't leave the toner on your hair for half an hour like the box says because that's too long especially if you already have light hair. (Unless of course you want it to be a lavender colour!) My hair is two different colours because the bottom of my hair was originally lighter than the top. So if you have really light hair your hair will look like how mine looks at the bottom, and if your hair is a brassy orange/yellow it will look like how it looks at the top! Pretty simple, right?
If you're using this toner to achieve white hair you will definitely get there as long as your hair is relatively light already.