Two kinds of color order systems-Type I (Munsell system, its representative) and Type H (Swedish Natural Colour System (NCS), its representative)-are widely used for systematic ordering of object colors. The reasons are clarified why two kinds of systems are necessary. The reasons are (1)the problem caused by the use of the present CIE photometric and colorimetric system and (2)the neglection of chromatic strength (CS) of each hue to perceived color attributes. In addition, a comparison is made between NCS and Nayatani Theoretical (NT) systems, which are in the same category of color order system-Type H. The importance of CS function is clarified in the comparison of the three color order systems studied. Special features of the three color order systems (Munsell, NCS, and NT) are described in detail. (c) 2005 Wiley Periodicals, Inc.