1. An image processing apparatus, comprising: an image region determining unit configured to determine a colored region where an inside of an outline is filled with a single color from color image data; an image color determining unit configured to determine a color of the colored region determined by the image region determining unit; a boundary determining unit configured to determine a boundary between the adjacent colored regions based on brightness and saturation of colors of the colored regions; and a change processor configured to change the colored region determined by the image region determining unit to a patterned region with a pattern different in each of the colors determined by the image color determining unit, when the colored regions with different colors are adjacent, change the adjacent colored regions to the patterned regions, and when differences in brightness and saturation of the adjacent colored regions whose boundary has been determined by the boundary determining unit are below a threshold, change the adjacent colored regions to the respective patterned regions.