V, B − V colour-magnitude diagram of NGC 1851 (grey circles). Superimposed as larger blue and orange filled circles are the giants investigated in this study. The blue symbols are for the metal-rich subsample stars and the orange symbols are for the metal-poor stars (see the text for the description of how the stars were separated into these subsamples). The photometric data were taken from Stetson et al. (2019); namely, the earlier database version corresponding to the one presented in Monelli et al. (2013).