An image processing method which converts a color image into a gray image, the image processing method comprising: determining a plurality of representative colors of an RGB color space with an image processor; storing a lookup table comprising the representative colors and first gray values corresponding to each of the representative colors by assigning the first gray values to the representative colors according to an NTSC color order of black, blue, red, magenta, green, cyan, yellow and white to reflect brightness of the representative colors which a user perceives with the image processor; and assigning a second gray value to an arbitrary color of the color image using the lookup table according to a degree of similarity between the arbitrary color of the color image and the representative colors and converting the color image into the gray image with the image processor, wherein the assigning first gray values to the representative colors comprises assigning first gray values such that the target brightness ranges, which correspond to ranges between the first grey values, are regular for each target brightness range; and wherein assigning the second gray value to an arbitrary color of the color image comprises applying weights to the first gray values assigned to the representative colors by giving greater weight to the first gray values corresponding to the representative colors which have greater similarity with the arbitrary color, wherein the assigning of the first gray values to the representative colors comprises compensating for a characteristic of brightness output of a printing apparatus which performs printing of the converted gray image. determining a plurality of representative colors of an RGB color space with an image processor; storing a lookup table comprising the representative colors and first gray values corresponding to each of the representative colors by assigning the first gray values to the representative colors according to an NTSC color order of black, blue, red, magenta, green, cyan, yellow and white to reflect brightness of the representative colors which a user perceives with the image processor; and assigning a second gray value to an arbitrary color of the color image using the lookup table according to a degree of similarity between the arbitrary color of the color image and the representative colors and converting the color image into the gray image with the image processor, wherein the assigning first gray values to the representative colors comprises assigning first gray values such that the target brightness ranges, which correspond to ranges between the first grey values, are regular for each target brightness range; and wherein assigning the second gray value to an arbitrary color of the color image comprises applying weights to the first gray values assigned to the representative colors by giving greater weight to the first gray values corresponding to the representative colors which have greater similarity with the arbitrary color, wherein the assigning of the first gray values to the representative colors comprises compensating for a characteristic of brightness output of a printing apparatus which performs printing of the converted gray image.