The results of (1) two experiments by Guth et al. and (2) an experiment by Nakano et al. on additivity-law failure were estimated using a simple formula for predicting the brightness/luminance ratio effect on the variable chromatic color method. The special feature of these experiments was to test additivity-law failures on mixture colors whose two component colors were mixed at varied mixing ratios. Agreement on additivity-law failures between the experimental results, and the corresponding predictions was found to be remarkably good for trend and fairly good for magnitude for all the experiments analyzed.