1. A non-transitory computer-readable storage medium storing a control program for color calibration, the control program comprising instructions which, when executed by a computing device or processor, cause the computing device or processor to perform operations, the computing device or processor being configured to control a printing unit and a color measurement unit and communicably connected with a storage unit, the operations comprising: (1) creating an image of a color calibration chart including an array of patches of colors in a color calibration target, and using the printing unit to print the image of the color calibration chart on a print medium as one of a plurality of paper products belonging to a predetermined type of print medium; (2) obtaining data of color measurement of the color calibration chart printed on the print medium in (1) from the color measurement unit; (3) calculating a white-ground color difference being a color difference between color values of white color in the color calibration target and measured color values of a ground of the print medium; (4) creating a color correction LUT which makes measured color values of each patch color of the color calibration chart, obtained by the color measurement in (2) close to color values of a corresponding color in the color calibration target; (5) adjusting the color correction LUT by repeating a feedback adjustment of the color correction LUT a predetermined number of times, the feedback adjustment including (A) creating an image of a corrected color calibration chart including an array of patches of colors in the color calibration target corrected with the color correction LUT, and using the printing unit to print the image of the corrected color calibration chart on a print medium as the one of the plurality of paper products belonging to the predetermined type of print medium, (B) obtaining data of color measurement of the corrected color calibration chart printed on the print medium in (A) from the color measurement unit, (C) calculating color differences between measured color values of patch colors of the corrected color calibration chart, obtained by the color measurement in (B) and color values of respective colors in the color calibration target, and obtaining a representative color difference of the color differences, and (D) creating a color correction LUT which makes measured color values of each patch color of the corrected color calibration chart, obtained by the color measurement in (B) close to color values of a corresponding color in the color calibration target; (6) choosing a smallest value among the representative color differences obtained by a predetermined number of repetitions of the feedback adjustment in (5), as a resulting color accuracy for the one of the plurality of paper products; and (7) performing (1) to (6) on at least one print medium as another or others of the plurality of paper products belonging the predetermined type of print medium, to create a database to be used for a judgement of a necessity for a feedback adjustment of a color correction LUT, for the predetermined type of print medium, the creating the database including, associating, with the predetermined type of print medium, the white-ground color difference and the resulting color accuracy both obtained for each paper product belonging to the predetermined type of print medium, to obtain a mathematical relation which gives from a certain white-ground color difference a corresponding resulting color accuracy, creating the database on a basis of the mathematical relation, and storing the database into the storage unit.