1. A method for dividing an image region of a color image according to colors, comprising the steps of: (a) setting a plurality of representative colors; (b) calculating plural angle indices for each pixel color in the color image, the plural angle indices for a pixel color representing angles between an individual color vector of the pixel color and representative color vectors of the plurality of representative color vectors in a predetermined color space of at least two dimensions; (c) calculating plural distance indices for each pixel color in the color image, the plural distance indices for a pixel color representing distances between the pixel color and the plural representative colors in the color space; (d) calculating plural composite distance indices for each pixel color in the color image based on the distance indices and the angle indices, the plural composite distance indices being associated with the plurality of representative colors, respectively; and (e) classifying pixels in the color image into plural representative color regions associated with the plural representative colors, according to the composite distance indices, thereby dividing the image region of the color image into the plural representative color regions.