[Problem] To provide a technique for calculating the psychophysical value of a color corresponding to an arbitrarily defined tone by modeling a tone concept defined in the PCCS (Practical Color Co-ordinate System).[Solution] A method for, regarding multiple types of tones defined in the PCCS, generating a definitional equation for each saturation to which each of the tones belongs, wherein a computer acquires the Munsell values of colors belonging to respective multiple tones belonging to the same saturation among the multiple types of tones defined in the PCCS, the computer stores the acquired multiple Munsell values in a predetermined storage device, the computer converts each of the multiple Munsell values stored in the storage device into a value in a predetermined color space composed of two axes of values representing lightness and values representing vividness, and the computer performs a predetermined regression calculation on a point group projected to the predetermined color space to find a regression equation passing through the origin of the predetermined color space.