A method for producing a color conversion table by which a color image signal to be handled in an image input system is converted to a N bit color image signal to be handled in an image output system, comprising the step of producing a color conversion table from which a N+.alpha. bit color output value, exceeding a color gamut of the color image signal to be handled in the image output system can be read out corresponding to a color input value of the color image signal to be handled in the input system.