When highlighting hair, the natural or artificial color-pigments where reduced step-by-step and changed in a way so they lose their color. The addition of H2O2 is necessary as this, along with the Bleach/De-colorizer, delivers the reactive oxygen needed for the oxidation. The alkalinity is necessary to swell the hair and control the activated oxygen in a way that the desired lift will be reached.
When you “bleach” white hair it becomes yellowish. The lipids and the colorless keratins get oxidized and become opaque.
Learn more about Color Theory here.