The present study describes the usefulness and importance of chromatic tone concept on object colors. It is clarified that the concept of a tone category consists of the same perceived lightness and the same degree of vividness of chromatic object colors in the tone irrespective of hue. Prediction equations are given to color attributes on perceived lightness and degree of vividness. They clearly show different functions on metric lightness and metric chroma on the two color attributes. It is also clarified that the theoretical opponent-colors system by the author (NT system) gives a basis for defining the tone concept, perceived lightness, and degree of vividness. The results of the present study are useful for understanding fundamental color notion "tone" which is important both in the fields of colorimetry (fundamental color-perception study) and color design (practical application). In addition, attributes of equivalent whiteness-blackness [W-Bk]eq and equivalent chroma Ceq are proposed. (c) 2005 Wiley Periodicals, Inc.