1. An image processing method for converting a first image that is expressed in a colorimetric system made up of a plurality of color components and comprises a plurality of pixels each having color information corresponding to one of the color components into a second image that comprises a plurality of pixels each having at least a common type of color information, comprising: a color judgment step that includes a color difference information generation step in which color difference information is generated for a processing target pixel in the second image by using color information in the first image through one of at least three different methods and a color evaluation step in which a color evaluation is individually executed for each of processing target pixels in the second image by using the color difference information; and an image conversion step in which the first image is converted to the second image based upon results of the color evaluation executed in the color judgment step.