C1 colour-colour diagram sensitive to metallicity and corrected for extinction. The histogram shows the distribution of a random subset of 5112 giant stars with [Fe/H] < −2 dex from Huang et al. (2022) with a total fractional flux uncertainty in the C1 bands of below 0.02. The colours refer to their photometric iron abundance estimates with the scale on the right-hand size. The round symbols indicate the 48 stars (not only giants) from Pristine discussed in Sect. 5.4 on the same scale using their spectroscopic estimates. The x and y axes are equivalent to CaII and MgH indices, respectively.