Foiling roots- going from 6A to 9N

update- I did it and I mixed 2 oz of Ion 9A with 1/4 oz of Ion 9V with a 30 volume developer. My target color was a 9N, but I knew it would pull warm, so I used the A and V to control and it looked GREAT! I was just a little apprehensive about the ratios of the Ash and the Violet, but I went with Claude's sticky advice. I just knew she wanted some warmth due to her desired 9Natural being slightly warm, but I was afraid with going 3 levels that it would have to much warm left in the end, so that is why I went with all cool controlling series and no natural. So, thanks Clyde for the sticky!