From left to right:F814W-(F275W − F814W) and F814W-C_{(F275W, F336W, F438W)} diagrams for a synthetic sample with −1 < [Fe/H] < −0.85) similar to the one shown in Fig. 5. The blue and red fiducial lines shown in each panel as solid lines (determined as described in Sect. 2) highlight the different widths of the RGB in the two diagrams. The width (in magnitudes) of the horizontal axis of the two figures is the same.