Diagnostic RV–[Fe/H] plane, metallicity histogram and Kiel diagram used to isolate cluster members for Gran 1 (upper row) and Gran 4 (lower row). Left panels: RV–[Fe/H] plane for all the MUSE-extracted stars in the Gran 1 and Gran 4 fields. The box was drawn to select the cluster members using the individual RV and metallicity values. Red crosses represent stars outside the calibration parameter space, i.e. stars in the RGB tip or HB/SGB stars, the RV of which coincides with the one derived for the cluster, but with no reliable stellar atmospheric parameter determination. Middle panels: metallicity histogram for cluster-selected stars. The highlighted hatched-filled histogram contains the cluster members with reliable metallicity measurements within the box. Right panels: Kiel diagram for the field and cluster stars in grey and green, respectively. A PARSEC isochrone was drawn using the same derived metallicity with an age of 10 Gyr. Number of members considers all the valid RV cluster stars, i.e., members within the highlighted box. The error bar located in each panel represents the mean uncertainty for each corresponding parameter.