To form a lookup table which makes the color expression of a dark space proper in color printing where no black ink but two or more colored inks are used.

The formation of the LUT converts input colors I in a first coloring system into output colors O in a second coloring system whose components are represented by the volumes of the inks constituting an ink set. Two or more colored inks which are capable of realizing an achromatic ink through a process where the colored inks are used as combined are set up as the ink set. The minimum intensity color expression ratio is decided as the component ratio of the output colors O set corresponding to the minimum intensity input color out of the input colors I. A correspondent relation between the input colors I and the output colors O is specified so as to enable the output colors O whose component ratio is the minimum intensity color expression ratio to be set corresponding to the above minimum intensity input colors.

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