A document image processing system includes an extraction portion, an estimation portion, a calculation portion, a substitution portion, and a generation portion. The extraction portion extracts first and second document elements of an inputted document image and a preprint data. The estimation portion estimates first and second representative colors due to first and second document elements in a color space, respectively. The calculation portion calculates first and second planes to separate the color space into first and second sub-spaces which each includes each of the first and second representative colors, respectively. The substitution portion substitutes a color of a first pixel of the first document elements with the first representative color and a color of a second pixel of the second document elements with the second representative color. The generation portion subtracts each of the substituted first pixel from each of the substituted second pixel to generate a difference image.