To generate a color separation table for an image forming apparatus which has relatively dark/light color materials with high/low relative densities, a gamut boundary for limiting the use of the relatively dark color material is set. One of relatively dark color materials is selected, and at least a grid point which satisfies a condition that the color material value of the selected relatively dark color material is nonzero and a color value to be reproduced based on color material values set to the grid point exists in a region brighter than the gamut boundary is determined. The color material value of the selected relatively dark color material set to the grid point which satisfies the condition is replaced by the color material value of the relatively light color material.