Residuals between synthetic photometry computed on three spectral libraries and the corresponding Gaia DR2 magnitudes as a function of the G (left), G_BP (centre), and G_RP (right) magnitude. Red triangles represent 43 sources from the CALSPEC database, green squares represent 81 stars taken from the Stritzinger spectral library, and blue dots show 40 objects from the NGSL library. 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.