Relationships between the photometric passbands V, P, and G, obtained by modelling f(λ) with A- to M-type synthetic stellar spectra extracted from the POLLUX database. Red polynomials are derived from Eqs. (8)–(10), and are applicable within the range 4000 ≤ T_eff ≤ 15000 (top frame) and −0.227 ≤ G_BP − G_RP ≤ 4.524. Gaia DR2 polynomial is based on Landolt stars observed with Gaia and is applicable within the range −0.5 ≤ G_BP − G_RP ≤ 2.75. It has a scatter of 4.6 × 10⁻² mag (see Busso et al. 2018). The relationship between T_eff and our synthetic G_BP − G_RP color index (bottom frame) is consistent with the color–temperature relations published in Andrae et al. (2018).