Great question! I've been looking for a solution too. I tried a conditioner that is meant to take brass out of blonde hair but it didn't do anything. I can't find anything for brown hair. But in theory the solution should be simple enough. All we need is a dye which is the exact opposite colour of the colour we want to remove and use that for a few minutes. If i's still brassy, repeat for another few mins until you get the brassiness out. I am thinking some shade of blue would do it. But where do we get a pure blue hair dye? If we could get it, we could mix it up with a weak developer and comb it through the hair for a very short time. Or add a little to our normal colour brown dye and do it that way.