Several color difference formulas were examined with the data by Davidson and Friede and by Thurner and Walther. The correlation between formulas and visual assessment is only moderate. The differences in the correlations of the individual formulas are too slight to give a reliable answer to the question which of the formulas are superior to the others. It is also still an open matter whether a formula is superior to the judgement by an average observer. Besides possible unreliabilities in the data there may be a fundamental difference between acceptability and perceptibility. New and very extensive investigations are therefore necessary.