I would suggest getting a professional colorist to dye roots only, and then just a heavy highlight, not all roots. I do this every four to six weeks, for about the last six years. Find someone that can do it without putting a toner on after the bleaching. Someone that can get to the platinum color without yellow, then you won't have brassiness later. Toners can cause brassiness. My colorist uses 40vol for about 40 minutes, but then we are all different. You might try having someone do a test on some of your underneath hair to see how long to leave it on. I have a dark , natural underneath panel that I never bleach. Makes me have interesting up-do's.
The blonde color suits you, but I did smile at the purple!