with the MS region below the knee. This fact is quite expected because the F110W$-$F160W color of M-dwarfs are significantly affected by the content of oxygen and we assumed the same value of \[O/Fe\] for the three populations, in contrast with what is inferred from spectroscopy (e.g.Grundahl et al.2002; Yong et al.2003, 2005, 2013; Carretta et al.2009).

To account for light-element abundance variations, we identified a series of fifteen points along the MS that span the magnitude interval between $m_{\rm F160W}=15.5$ and $m_{\rm F160W}=23.0$ in steps of 0.5 magnitudes and calculated for each of them four synthetic spectra. To calculate all the spectra we assumed \[Fe/H\]=$-1.61$, \[$\alpha$/Fe\]=0.4 and the values of effective temperature and gravity inferred by the best-fit isochrones and a microturbolent velocity of 2 km s$^{-1}$. Specifically for each population, we calculated a reference synthetic spectrum with solar carbon and nitrogen abundance and with \[O/Fe\]=0.4 and a comparison spectra with the corresponding average abundances of C, N and O derived from spectroscopy. Specifically, we used (\[C/Fe\], \[N/Fe\], \[O/Fe\])=($-$0.25, $-$0.11, 0.65), ($-$0.45, 0.92, 0.43) and ($-$0.70, 1.35, 0.03) for population A, B, and C, respectively (see Table 2 from Milone et al.2013 and Yong et al.2005, 2013, 2015).

Synthetic 