Nayatani-theoretical (NT) color order system is a Hue-Tone color order systems. It can be used for determining surpface colors with the same Tone(equiperceived lightness and equiperceived chroma) irrespective of hues under reference illuminant. The fundamental structure of NT system uses Munsell attributes HV/C for the easiness of its use. Some deviations caused by the approximation on chroma scale are naturally expected in NT system, although the formula used for deriving equivalent lightness V-eq has the same structure as that for deriving L*(epsilon q), using the corresponding CIELUV formula. The present article discusses the degree of approximation, and confirms the goodness of approximation. NT system can be used as it is, for the field of artistic color design (complete accuracy is not required). The corrections, however, should be introduced in NT system, when higher accuracy is required. (c) 2007 Wiley Periodicals, Inc.