Grays of input image signals in a set of input three-
color image signals are compared to determine a maximum input gray (Max), a middle input gray (Mid), and a minimum input gray (Min) among the input image signals and to assign an order index to the
signal set. The Max, Mid, and Min are gamma-converted into Gamma(Max), Gamma(Mid), and Gamma(Min), respectively, and a maximum output gray (Max'), a middle output gray (Mid'), a minimum output gray (Min'), and an output white
signal (W') are determined. The four
color image signals are generated from the Max', Mid', Min', and W' based on the order index.