A u − g vs. g − z colour–colour diagram for solar metallicity (bottom panel) and − 2.0 (upper panel). The solid lines are the lines of constant temperature and constant surface gravity defined by the Castelli & Kurucz (2003) grid. The black dots correspond to our ATLAS models, that sample points within the Castelli & Kurucz (2003) grid that correspond to the parameters of the models in the CIFIST grid. The Castelli & Kurucz (2003) grid has steps of 250 K in Teff and 0.5 dex in logg. The red squares are the “3D-corrected” colours.