Santa White Hair
A: The short answer is to wait sufficient time and hope that your genetics will cooperate.
The long answer is that you can't really make the hair "pure white". The snowy white color of Santa's hair comes from the fact that he is 100% gray. His hair lacks any pigmentation whatsoever and is therefore slightly translucent.
What you MAY be able to achieve is to lighten the hair to the lightest possible level and tint it using a platinum blonde tint to eliminate any yellow cast that might remain. I say MAY because it depends a lot on the color of your natural hair.
Going from black or dark brown to "Santa White" would be nearly impossible without the likelihood of completely destroying the integrity of the hair.
It's easier if you are starting from a lighter blonde shade, and if the hair is already mostly gray, the results will look better still. You simply must remember that the darker your hair is the more it must be lightened, and the more the hair is lightened, the more the hair is likely to be damaged in the process.
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See also:
Shades of gray hair
How can I speed up the change of my hair from gray to white?
My white hair has a yellow tinge. How can I solve this?
Can hair turn white overnight from stress or shock?