1. An image processing device which converts color image data representing a color image into monochromatic image data, the image processing device comprising: a pattern determining unit that determines a monochromatic pattern representing a shape corresponding to a color of the color image by a monochromatic concentration; a pattern concentration determining unit that determines a background concentration of an area of a background of the shape in an image area of the monochromatic pattern; and a monochromatic image data generating unit that generates the monochromatic image data of the monochromatic image to which the monochromatic pattern having the background concentration is applied in the image area corresponding to the color image, wherein the pattern concentration determining unit changes the background concentration according to the color of the color image.