A new model of turbid medium theory is proposed that combines the best parts of the opaque forms of Kubelka-Munk single and two constant models. This model introduces an impurity index, a spectrally nonselective scattering coefficient for each chromatic component. The new model is shown to have a convex, linear dependence on colorant concentration so that it can be used for colorant identification in high-resolution images of works of art while offering an extension to the single constant model that can predict the absorption and scattering of paints, both in mixture and pure (masstone) forms. The model was tested and validated using 28 matte acrylic dispersion paints, of the type used in modern painting. These paints had a wide range of absorption and scattering characteristics. (C) 2016 Wiley Periodicals, Inc.