Undercolor reduction is performed to a color image that is to be marked to reduce an amount of marking material used to mark underlying colors in overmarked pixels where a top color is marked over at least one underlying color. Information is generated to designate the overmarked pixels. This information may be generated as tags. A raster image is created in which both the underlying and top colors are included in the overmarked pixels. The image data of the raster image is modified such that an amount of marking material used to mark the at least one underlying color in each overmarked pixel is reduced.