It is important to understand that Pink Platinum is still the Colour Restore Iced Platinum formula, and is an anti-yellow toner, designed for use on yellow or brassy hair. If your hair is already a neutralised platinum shade and Pink Platinum is used, there will be no yellow in the hair for the product to neutralise and the hair will display as lilac. The lilac is seen because the acid-violet molecule is designed to neutralise yellow. So, in a brassy base, this acid-violet molecule counteracts the yellow and vanishes creating white and leaving only the pink molecule to display. However, if someone with neutralised white hair uses the product, there is no yellow for the acid-violet pigment to neutralise and it displays in the hair alongside the pink molecule. The pink molecule is derived from red and the acid-violet derived from blue. Therefore, you mix these two molecules together and obtain violet.
If you are faced with this issue, just keep washing the hair with a generic regular shampoo to start allowing the underlying brassy tone to show through. At that point, the acid violet pigment will vanish, because the pigment will try to start neutralising the yellow. Once this happens, the pink tone will be seen. From this point, you should only apply Colour Restore Pink Platinum when the hair is displaying a visible brassy tone that requires neutralising.