In image processing in which each small area included in color image data, which is represented by C, M, Y, and K, is expressed by a predetermined number of representative colors, values of color parameters are obtained for a small area from the values of the color parameters for C, M, Y, and K for each pixel in the small area, so that the values of the color parameters have degrees of variation being compensated to become smaller in the group as the value for black becomes larger. One color parameter to be targeted is selected (#104) based on the degree of variation in the values of individual color parameters in the group. A reference value to be used for grouping the pixels is obtained based on the relationship between the values of the color parameter to be targeted in the group and the quantities of pixels in the group. Based on the reference value, the pixels are divided into groups (#106), and a representative color for each group is obtained (#109).