I have bleach blonde hair with med brown roots, I would like to keep my hair blonde but wanted to try something else other than bleach that isn't going to be so harsh, I have ordered a lightest blonde high lift colour and was wondering if this would leave my hair brassy against the dark brown lights I'm putting in too?

Dark brown lowlights against very light blonde hair will look very dramatic, and possibly harsh. If you want something a little more subtle, you could go for a light blonde canvas with light brown, or dark blonde low-lights.

If you needed bleach to get your hair the color it is now, then high-lift tints won't work. Medium brown is usually level 4 or possibly 5. Bleached hair is almost always level 10; occasionally it might be level 9. In any case, you're trying to lift four or five levels. High lifts are good for an absolute maximum of three and a half levels. After that, it simply gets too brassy. In addition, high-lifts are almost always more damaging than bleach.