Comparison between synthetic photometry computed on external high-resolution spectra and that computed on ECS in the Johnson-Kron-Cousins photometric system. Data are from SPSS (open red circles), PVL (blue crosses), and NGSL (open green triangles) samples. The comparison is shown as a function of G magnitude (left) and colour index G_BP − G_RP (right). Two horizontal lines at ±0.02 are shown for reference.