Comparison between Landolt standard BVRI magnitudes and synthetic BVRI photometry in the Johnson-Kron-Cousins system computed on externally calibrated BP and RP spectra for a sample of 32 781 sources selected from the Landolt collection of standards. Left: residuals as a function of magnitude. The red lines represent the smoothed median distributions. Right: residuals as a function of G_BP − G_RP colour for sources with G < 17.