A microcomputer equally divides the circumference of a RGB circle into 6×n (n is an integer of 1 or more) parts, and calculates a RGB value of each divided color. (255, 0, 0) is stored as a reference RGB value of a reference color in a ROM in the microcomputer. The microcomputer 3 converts the reference RGB value depending on an angular difference on the RGB circle between a designated color whose RGB value is to be found and the reference color, and assumes the converted RGB value as a RGB value of the designated color.