Simulations of absorption and colour excesses using the Basel-3.1 synthetic spectral library (Westera et al. 2002). Left: relationship between absorption in the V (grey) and G (cyan) passbands. For the polynomial fitting in Eqs. (1) (blue line) and (2) (orange line), we used only the magenta points (sources with reliable E(G_BP – G_RP) estimations). Centre and right: Near-infrared 2MASS colour excesses as functions of E(G_BP – G_RP) Centre: 2MASS E(J – H) (grey) and E(H – K_s) (cyan) and lines show the polynomial fits in Eqs. (3) and (4). Right: excess in the combined optical-infrared colour E(G_BP – K_S). The blue line shows the polynomial fit given in Eq. (5).