1. An image processing method for converting a first image which is expressed in a colorimetric system made up of a plurality of color components and is constituted of a plurality of pixels each holding color information corresponding to a single color component to a second image constituted with a plurality of pixels all holding color information corresponding to at least one common color component, comprising: a first color information generation step in which color information corresponding to a color component different from the color components making up the calorimetric system used to express the first image is generated by using color information corresponding to the plurality of color components making up the first image; a second color information generation step in which color information corresponding to a color component matching a color component in the colorimetric system used to express the first image is generated by using color information corresponding to one color component in the first image; and a color information composition step in which, depending upon a pixel position, the color information having been generated through the first color information generation step or the color information having been generated through the second color information generation step is used selectively, or the color information having been generated through the first color information generation step and the color information having been generated through the second color information generation step are both used for weighted composition.