Responsive to image data of three colors, six hue data are obtained, and then first comparison-result data and second-order terms each relating to one of the six hues, and second comparison-result data each relating to one of the six inter-hue areas are obtained. Matrix calculation is performed on the first comparison-result data, the second comparison-result data, and the second-order terms, using coefficients. By varying the coefficients, adjustment can be made to only the target hue or inter-hue area, without affecting other hues and inter-hue areas. Thus, the six hues and six inter-hue areas can be varied independently, and the large-capacity memory is not required. In addition, gray scale conversion is applied to the result of the matrix calculation, so as to compensate for the non-linearity of the output device.