These tests compare the photometry of Gaia DR2 with external photometry. We test the Gaia DR2 provided passbands by comparing the photometry computed using the Hubble stellar standards Calspec database to the observed photometry.
We compare Gaia DR2 photometry to the Hipparcos, Tycho-2, 2MASS (Skrutskie et al. 2006), the SDSS tertiary standard stars of (Betoule et al. 2013) and Pan-STARRS1 (Chambers et al. 2016) photometry, selecting low extinction stars only () using the 3D extincton map of Capitanio et al. (2017). We check the distribution of a mixed colour index Gaia magnitude minus the external catalogue magnitude in ordinate versus an external catalogue colour in abscissa. An empirical robust spline regression is derived which models the global colour-colour relation. The residuals from this model are then analysed as a function of magnitude, colour and sky position.