1. An image processing method of making a monochromatic image signal made up of image signal components representing image data on respective pixels from a color image signal made up of image signal components representing image data on respective pixels, the method comprising, the steps of dividing the color image signal into a plurality of blocks of a predetermined size, and determining the values of brightness and chromaticity of each block on the basis of the values of brightness and chromaticity of each pixel in the block represented by image signal components for the pixel of the color image signal, the step of determining whether a designated block is a block in a color border region of a type where two color regions which are small in brightness difference and large in chromaticity difference are positioned side by side on the basis of the difference in value of brightness between the designated block and neighbor blocks near the designated block and the difference in value of chromaticity between the designated block and the neighbor blocks, the steps of, when the designated block is determined to be a block in a color border region of said type, obtaining the values of brightness and chromaticity of at least two colors representative of colors forming the color border on the basis of the values of brightness and chromaticity of the neighbor blocks, and changing the value of brightness of each pixel in the designated block to a new value different from that represented by the image signal component for the pixel of the color image signal on the basis of the values of brightness and chromaticity of the pixel, the values of brightness and chromaticity of the respective representative colors and a preset order of priority of colors, and the step of making a monochromatic image by setting values of brightness of pixels in the block in the color border region to said new values and setting values of brightness of pixels in the blocks in regions other than the color border region to those as represented by the image signal components for the pixels of the color image signal.