I'd do a full head of fine bleach foils with 10v (so off scalp no irritation) and then go inbetween the foils with what ever root colour you think. If you want to go up a couple of shades use an ash to combat warmth. I lift loads of my ladies up with tint and will use hi lift on a percentage of white hair (not normally 100%) It won't cover but will blend, then you could tone the whole lot afterwards with a cool toner, flattening any root warmth as well as toning the bleach. You could use two seperate colours on roots and ends if you wanted. One of my favourite toners is champagne by leyton house with 1.5v it's lush and seems to sort all the warmth without the blue hue. Sometimes it's a work in progress and will take a couple of goes to get it right. Go for it!