When a monochromatic image is reproduced, a user can select desired mixing coefficients of red, green and blue in order to control gradation data for the reproduction of a monochromatic image as well as a monochromatic reproduction color. When a document is automatically determined not to be a color document, the mixing coefficients of red, green and blue are changed according to the type of the document. If a monochromatic copy mode is selected, the mixing coefficients can be set manually. Preferably, in order to help a user to select mixing coefficients, a plurality of images of monochromatic colors of various mixing coefficients of red, green and blue can be printed on a single sheet of paper.