1. An image processing apparatus comprising: an object type determining unit that, for each drawing object in input image data, makes use of a type and color information of the drawing object and determines whether the drawing object includes a color character group of a single or more characters; a background area extracting unit that, when the drawing object is determined to include the color character group, extracts a background area on which the color character group is formed in the drawing object for each color character group; a monochromatic conversion unit that receives specification of a main color as a printing color and specification of a color expression color, and, when the drawing object is determined not to include the color character group, generates monochromatic image data by performing monochromatic conversion of the input image data with a predetermined luminance value, and when the drawing object is determined to include the color character group, generates monochromatic image data by performing monochromatic conversion of the input image data with minimum luminance of the main color; a pattern generating unit that, when the drawing object is determined to include the color character group, obtains a hue of the color character group and, according to the hue, generates pattern image data to be added to the background area of post-monochromatic-conversion color character group; and a synthesizing unit that synthesizes the monochromatic image data and the pattern image data.