As the basic theory for color mixing in intensely light-scattering materials, the Kubelka— Munk single-constant/two-constant theories have played an important role in color science and technology. However, in practice there are some shortcomings. This paper checked the theories with a series of systematical experiments, data processing and analyzing, and has revealed several disagreements with the experimental evidence in the case of the additivity color-mixing law for turbid materials. Explanations on some of these problems are given, which are different from the conventional views.