As Fig. 2 but including also the observed and theoretical RGB sequences. The top panel displays the [Fe/H] = − 0.7 baseline simulation employing the BaSTI adopted BCs and colour transformations (ATLAS9). The bottom panel shows the result for the baseline simulation but employing the empirical Worthey & Lee (2011) BCs and colour transformations (solid line). The reddest and bluest limits of the RGB colours according to the errors on the Worthey & Lee (2011) transformations and BCs are displayed as dotted lines (see text for details).