Data in only three colors is printed in the three colors plus black. The coverage amount represented by the smallest intensity of the three colors is determined. When that amount is above a predetermined value at which observable grain becomes minimal, an exponent value is determined and a formula is followed. As an additional improvement, lightness is redefined on a model which favors less black at the lighter values and more balck at the darker values. Hue is to remain the same, ans is known from the original color data. The chroma is refined based on the original chroma, CMY, and CMYK full gamut boundaries. Since black, lightness, chroma and hue are known, the new three color values are found by known techniques.