Recently I learned about bleach bath. It suppose to lift about 1 or 2 levels without making too much harm. After bleaching the whole head to level 10, i used bleaching powder and 20 vol dev. to lift the roots. All I got was level 9. Then after toned, the roots was slightly yellowish. They need to be lifted one more level. Will bleach path work for this? Shall I apply it only on roots? Or in the whole head? Any other ideas?