Intrinsic CMD of the initial sample of LRS (top) and MRS (bottom) stars, divided in four [Fe/H] bins, coloured by the density of stars (logarithmic scale). The number of stars is plotted in each panel. The grey lines represent the cuts done to select MSTO and SGB, as is explained in Sect. 3.3. To guide the eye, we additionally plot in blue two isochrones representative of each [Fe/H] bin at two different ages.