Approximate relation between A_G and E(BP − RP) (labels of EXTRATREES training data) for PARSEC 1.2S models (Bressan et al. 2012) with 0 ≤ A_G ≤ 4 and 3000 K ≤ T_eff ≤ 10 000 K. PARSEC models use the extinction law of Cardelli et al. (1989). We see in panel a that most stars follow the relation A_G ~ 2 × E(BP − RP) (dashed orange line) while the stars highlighted in red behave differently. Panel b shows that these star with different A_G –E(BP-RP) relation are very red (i.e. cool) sources.