1. An image processing method for extracting a point positioned at a boundary between regions having different colors, comprising: using one or more computers to perform the steps of: a step A of setting a processing object region having a predetermined size and centering around a pixel that constitutes the color image to be processed; a step B of finding the magnitude of change in color difference in each of X and Y directions crossing perpendicularly in the pixel in the center of the processing object region using data of pixels in the region; a step C of finding the direction of a color parameter change generated based on the pixel in the center of the region in a processing object region in which the magnitude of the change in color difference in each of the X and Y directions found by the step B corresponds to the magnitude of the change in color difference recognized at the above boundary between the region having the different colors, based on at least one value of a color parameter among three independent color parameters showing color information of each pixel and determining that a vector showing the direction of the change corresponds to which quadrant in a coordinate system defined by the X and Y directions; and a step D of specifying a direction of the change in color in the central pixel in the processing object region processed at the step C, using the determination result at the step C and the magnitude of the color change in the X and Y directions found at the step B.