i've used t18 and love it, but it fades way too fast. i noticed there was a permanent dye called "Palest Ash Blonde" 10A/1030, the lightest dye you can get in the Wella brand. this seems like the closest thing i can get to t18 as a permanent dye, but will it last longer? anyone use it before? tried searching, and although theres lots for t18, nothing came up for 10a/1030.
My experience with the light Wella blonde dyes is that they also fade about as quickly as the toners if applying over bleached hair. They are both quite damaging, so I'd recommend a semi-permanent toner instead as your hair is already light enough to tone. It has about the same longevity, but doesn't do any damage as it doesn't require peroxide.
If you've already got a relatively pigmented purple shampoo, I'd just try leaving that on dry, clean, unconditioned hair for an hour or two and see if that works.