Distribution in the Pristine color–color space of the 23 known UMPs in common with the Pristine-Gaia synthetic catalogue. Stars are color-coded by their photometric metallicities [Fe/H]_phot. The blue and black lines are polynomial fits to the expected location of stars with [Fe/H] = −3.0 (blue) and [Fe/H] = −4.0 (black) based on the spectral synthesis described in Section 6.2 and also represented by star symbols in Figure 9. The gray points correspond to stars with −4.0 < [Fe/H]_phot < −3.99 and highlight the location of the region with [Fe/H]_phot = −4.0, as assigned by the Pristine model. Only two stars, represented by squares, are above this region and technically rejected by the [Fe/H]_phot > −4.0 cut recommended in Sub-section 7.3. Because these stars have large uncertainties, requesting only [Fe/H]_phot,84th > −4.0 would select them back in the sample of high-quality stars.