Lightening a dark box colour.

I initially wanted a beautiful light purple I had found online, the salon told me it would take three visits, 300+ dollars and two months of being blonde - not something I wanted. I decided to find a boxed colour and made the mistake of Vidal Sassoon deep violet on my head.. (It's a lovely product/shade, just too dark for my liking.) A week into it and I'm regretting the choice. Wondering if I can do anything to my locks to lighten it up to an ashy purple or something along those lines? Any help would be super appreciated!
Cheers