Trying to match salt and pepper roots

I've colored for years to cover my gray. I'm tired of the hassle of coloring every 3-4 weeks and would like to try and match my natural salt and pepper using wella t14 toner and then foil in some medium cool brown for the heavy pepper in my salt and pepper natural color. I am naturally light brown but have bleached to a level 7-8. Will this work and should I start applying the toner to my new growth (2") or do that last?