Left panel: the ratio of the number of stars falling on the Main Sequence (MS, stars) and the RC (circles) to those on the RGB, as outlined in Fig. 5, for both observations and models for different filter combinations. These ratios have been normalized to the observed MS/RGB ratio. Right panel: the χ² values for different combinations of filters for the set of models defined in Fig. 6, using a bin size of 0.15 in magnitudes and 0.25 in colour (see text for details). For this computation we have used only the stars in F2, with V,I,J,H and K_s photometry. Both panels: the colour code (only in the electronic version) is blue for V − I, green for V − K_s, yellow for I − K_s, black for I-H, magenta for J − K_s and red for H − K_s.