Ash is a green to green/violet based color (pigment). When a color says Warm Brown, it means it contains a certain amount of red/orange pigment in the mixture. When it says Ash Brown, it contains a certain amount of green or green/violet in the mixture. When the two are mixed together at equal proportions (one ounce of each color mixed with 2 ounces of peroxide), then one base color will counter-act the other base color leaving you with a "natural" pigment rather than one that is too red or one that is too green.