A newly modified opponent-colors theory is proposed, with the introduction of a concept of relative chroma. Its maximum value is different for different hues and is determined by the chromatic strength of each hue. The newly modified theory is able to clarify the following four points: (1) a contradiction found between color attributes (whiteness, w; blackness, s; and grayness value, gv) for different chromaticness values in the NCS color-order system; (2) the difference between a constant hue interval and its corresponding perceived hue change (or hue color difference) in the neighborhood of each of the our dual colors raised by Hard et al. (Color Res Appl 1996;21:180-205, 206-220); (3) further strong support of the existence and importance of the reference color gray; and (4) the importance of chromatic strengths of various hues used in color-appearance studies. The newly modified theory retains most of the merits found in the previously modified one relative to the Hering opponent-colors theory. (C) 2003 Wiley Periodicals, Inc.