C1 colour–colour diagram sensitive to global metallicity for a set of field stars. Lines in the figure represent the simulated photometry from BTSettl SEDs, plotting only solar alpha abundance ([α/Fe] = 0) for log g = 4. Dots represent the synthetic photometry derived from BP and RP spectra with a colour index showing the DPAC CU8 metallicity derived using the GSP-Phot algorithm (mh_GSP-Phot).