Comparison between 2MASS-K_s SBCRs from Paper II and theoretical surface brightnesses calculated from stellar MARCS atmosphere models defined in Table 1, i.e. with a log g varying with the effective temperature. The metallicity is considered to be solar. The shaded grey area corresponds to the 1σ confidence interval of the empirical SBCRs (corresponding in some cases to the thickness of the line). The lower panel shows the difference between the models and the corresponding SBCR (giants or dwarfs, respectively) in a fraction of the RMS of the empirical SBCR.