Metallicity estimation analysis. The top panel illustrates the CBDD of late stars with spectroscopic metallicity from Do et al. (2015) and Feldmeier-Krause et al. (2017b). We used 12 Gyr Giant branch isochrones with different metallicities from Bressan et al. (2012), Chen et al. (2015), Marigo et al. (2017) to convert T_eff into CBD values. The bottom panel shows the CBDD diagram of stars lacking spectroscopic metallicities and not classified as spectroscopic early. Metallicities are assigned using these isochrones. Both plots utilize the same colour scale and are restricted to stars brighter than 14.7, as this is where the presence of the RC, not included in our model, becomes significant. Stars encircled in grey represent those classified as likely early by the Bayesian method.