Colour – magnitude diagram (CMD) of the Pristine-Gaia synthetic (PGS) metallicity catalogue. The sample, after applying the quality cuts described in Sects. 3.1 and 3.2 is colour-coded in grey. Pristine-Gaia synthetic stars with [Fe/H]_PGS < –1.5 dex are shown with a density plot. There are two colour codes for the interlopers: dwarf stars are displayed in deep purple and isochrone-filtered sources in light purple. Left panel: before filtering, 9 432 286 stars. Right panel: after, 2 880 338 stars (Sect. 3.3). The blue curve corresponds to the dwarf and giant separation of Fernández-Alvar et al. (2021).