PURPOSE: To automatically execute a selecting work of a color enabling to display simultaneously, which should be depending on man-power up to the present, in a system in which an image display system can develop N colors in M colors.

CONSTITUTION: This method is provided with (l) a means for counting colors of an original image generated within a range of the number of colors of M colors at every dot, and counting the frequency of each color, (2) a means for selecting a combination of the most adjacent colors, in the case the number of colors whose frequency in not zero exceeds N colors by the counting means, and (3) a means for adding the color whose frequency is lower to the frequency of the color whose frequency is higher in the combination of the selected colors, and thereafter, setting the frequency of the color whose frequency in lower to zero, and determing to convert the color whose frequency is lower to the color whose frequency is higher, and this processing is repeated until the color whose frequency is not zero becomes below N colors by (2) and (3), and at the time point when it becomes below N colors, the color frequency is not zero is used as a display color, and the number of colors is converted by applying the color conversion determined by the processing of (3) to each color of the original image. In this regard, after the conversion is executed, the fine adjustment is executed if necessary.