Sorry for the loaded answer, but I have been told by two friends with experience, and after a failed research (due to multiple contradictory anwers), I got different anwers from two hair dressers too.
You can bleach it very light yellow, and then add something to make it white for a few weeks (but nothing to make it permanently white).
So, why can’t we? When bleach destroys the pigments in our hair thats making it brown or black or whatever color, what is it that’s left to make the hair seem yellow/blonde? I have been told that it is the natural color of keratine and other stuff that builds out hair, but that doesn’t make too much sense to me since older people usually have gray/white hair (which is due to the hair strands being transparent, afaik), and it is the same stuff.
Anyway, if there is some pigment in our hair that is keeping it yellow after bleaching, why can’t we remove that too?
You can bleach it pure white with enough bleach. I did this in high school, pretty much fried my hair but it was pure white. I was shaving it all off anyway so I didn’t care.
Keratin is transparent, but the way it disperses light it might result in white/whitish/grayish/yellowish color, depending on the density and orientation of fibers. When you bleach it and add a blue wash, you basically counter any possible yellow photons being reflected (since blue will absorb them).