Left panel: zoom in on the RGB of ESO540-G032 with a subset of Dartmouth isochrones overplotted. The isochrones have a step in metallicity of 0.1 dex, and they range from −2.5 dex to −0.9 dex in [Fe/H], where the metallicities increase toward redder colors. Right panel: the open circles indicate the metallicity errors as a function of metallicities based on Dartmouth isochrones, while the error bars indicate the standard deviation of the metallicity errors.