PURPOSE: To exactly and efficiently execute color coating and formation of plain halftone films with a computer by providing a display color changing means and approximate color display means and displaying colors which are distinctly discriminatable in visual sensation on a display even if registered dot percentage differences are small.

CONSTITUTION: The approximate color display means 13c displays the colors approximate to the colors formed by the inputted dot percent data on the display 17 and the inputted dot percent data is stored in a memory means 18. The colors displayed on the display 17 is adequately changed without changing the registered dot percentage by a display color changing means 13d. A color pallet separated by the displayed colors is displayed. A color separating means 14 coats the displayed colors successively on the color pallet for the image which is extracted from the image data stored in the memory means 18 and is displayed on the display 17 thereby forming the color sepn. data. The color sepn. data is stored in the means 18 and is then outputted. An original plate forming means 1 forms the color sepn. plates for late making for printing.