Experimental results on additivity-law failure observed by direct brightness matching are estimated for various combinations of two spectrum colors. The estimations are made by using the prediction equation of the Brightness/Luminance (B/L) ratio effect on chromatic colors, which is based upon the Variable Chromatic Color (VCC) method for the effect. The predicted results confirm the existence of the two types of additivity-law failures called enhancement or cancellation, already reported by several researchers. The prediction equation also clarifies that the effect of additivity-law failure does not change for a wide change of adapting luminance used in observation Both B/L effect and additivity-law failure can be estimated quite nicely by the same prediction equation without making any modification to it. The Helmholtz-Kohlrausch effect can imply bath effects in its wide definition. (C) 2000 John Wiley & Sons, Inc.