Residuals between synthetic photometry computed on the NGSL, CALSPEC, and Stritzinger libraries and the corresponding Gaia DR2 magnitudes as a function of the observed G_BP −G_RP colour. Symbols are the same as Fig. 24. Horizontal green and red lines represent a linear fit (with sigma-rejection clipping) to data with G > 10 and the ± 1σ region. Top:synthetic magnitudes computed by convolving the source SEDs with the DR2 passbands and normalising by the photometric zeropoint. Bottom: synthetic photometry obtained with the REV passband set.