1. An image processing apparatus comprising: an inputting unit that inputs a plurality of component images, each of which comprises a plurality of pixels, the pixels having attribute information which indicates whether the pixels are achromatic or not; a combining unit that combines the plurality of component images into a composite image, the attribute information of the pixels of the plurality of component images being combined into attribute information of the composite image; an attribute appending unit that appends an identification number to the attribute information of each of the pixels, the identification number indicating which of the component images each of the pixels having the identification number belongs to; an analyzing unit that analyzes, for the pixels which have a common identification number together, the attribute information of the pixels of the composite image to determine whether the pixels which have the common identification number are achromatic or not; and a processing unit that converts a color component of the pixels which are determined to be achromatic into a black monochromatic color.