In most cases, O3 & RO1 used individually or mixed together will work to filter away the brown that our eyes sees. In reality, the discoloration is blue, green, a combination of both OR even yellow/green. For larger areas that appear yellow/green, I recommend "toning" first with VR1 and then spot correcting the darkest remaining points using the O3 & RO1 combination. I suggest also having Y1 on hand. After filtering the "brown" spots often times you will see a purplish remnant that can then be neutralizes with the Y1 (yellow 1).