How do I get my hair gwen stefani or christina aguilera white?

My hair is very very light blonde, and I went to the hairdressers and asked for platinum blonde hair so they bleached and toned it with del lorenzo brand products but its not fully white like gwen or christinas or lady gagas hair. Its like a silvery pearl blonde. I tone it all the time with a purple toner and I also use Lee stafford purple shampoo, but I just cant reach the fully white hair.