1. An image processing apparatus comprising: storing means that stores data of a first color group including at least one color in association with data of a second color group made up of colors obtained by allowing an image output apparatus to output the colors of said first color group, said colors having a predetermined relationship with the colors of said first color group; and calculating means for inputting from the outside data indicating a color group obtained by allowing said image output apparatus to output the input color group, calculating a lightness error, chroma error and hue error using said data and the corresponding color group data corresponding to said input color group stored in said storing means and further calculating a color reproduction index for color matching using these errors.