A method when expressing gradation for a color image including binarizing tone values of first image data to produce second image data including pixels each having colors with respective binarized tone values, classifying pixels of the first image data into a plurality of first pixel groups and specifying a pixel where a color centroid is positioned for each color in each first pixel group, classifying pixels of the second image data into second pixel groups PXG2 corresponding to the first pixel groups and calculating the number of light pixels for each color in each second pixel group, producing third image data from the second image data, information related to the color centroid, and information related to the number of light pixels, and producing fourth image data by changing the position of a light pixel with the position of a dark pixel in the third image data.