In a high lightness region of a color hue corresponding to a basic color material, the high lightness region ranging from a lightness level corresponding to the maximum color saturation level that is reproducible by the basic color material to the highest lightness level, color separation data corresponding to the basic color material is generated. In an intermediate lightness region whose lightness level is lower than that of the high lightness region, color separation data corresponding to the basic color material and two color materials (one of them is preferably is a particular color material) that have color hues close to that of the basic color material is generated. This provides the advantage of enlarging the color gamut in the low lightness region of a basic color hue.