Directcomparison of the empirical extinction coefficient k_G as a functionof (G − K_S)0 for A₀ = 0.1, 3.5, 13.3 mag (top panels) and A₀ for (G − K_S)0 = 2.58, 1.79, 0.98 (bottom panels, which corresponds to T_eff = 4020, 5080, 6500 K). A₀ = 3.5, 13.3 mag are the limit values for the MS and RG samples, respectively (see Table 3) while (G − K_S)0 = 2.58, 1.79 are the maximal colour of LAMOST and the minimal colour of APOGEE, respectively. (G − K_S)0 = 0.98 is a sample case to show the behaviour at low colour indexes (i.e. high temperatures). Dots show the k_G mean value while the shaded area shows the 95% interval of confidence (see Sect. 5). Colours correspond to the red giants (RG, red), the main sequence (MS, blue) and unified sample (RG + MS, green). Black triangles and magenta squares show the theoretical k_G coefficient computed with the Gaia pre-launch (NOMINAL, Jordi et al. 2010) and Gaia G-DR1 (M.A. 2017, Maíz Apellániz 2017) passbands, respectively.