A color separation section for converting colorimetric values into color separation values adopts a neural network. When a color image output device for outputting a color image on the basis of color separation value signals is used, the color image output device to be used outputs a standard color sample having known color separation values. The color sample is colorimetrically measured by a colorimetry device to obtain colorimetric values of the color sample. The neural network executes learning to have conversion characteristics for converting the colorimetric values into corresponding color separation values. An object to be reproduced which has a required color is colorimetrically measured by the colorimetry device. Colorimetric values obtained by the colorimetry processing are converted into color separation values using the neural network of the color separation section. The color image output device outputs a target color based on the converted color separation values.