on the naive estimator method (Silverman 1986). In a nutshell, we first defined a series of magnitude bins of width $\nu=0.25$ mag over a grid of $N$ points separated by steps of fixed magnitude ($s=\nu/5$). Then, we calculated the medians of the $m_{\rm F110W}-m_{\rm F160W}$ color distribution and of the F160W magnitude distribution of all the MS-A (MS-C) stars in each bin. Finally, we smoothed the median points by using the boxcar average smoothing, where each point has been replaced by the average of the three nearby points. The fiducial lines are used to derive the verticalized diagram plotted in the middle panel of Fig. \[fig:pratio\], where the abscissa is calculated as: $$\Delta_{\rm F110W,F160W}= [(X-X_{\rm red~fiducial})/(X_{\rm blue~fiducial}-X_{\rm red~fiducial})]-1$$ with $X=m_{\rm F110W}-m_{\rm F160W}$. To account for the contamination from foreground and background field stars, we used the population-synthesis code Trilegal (Girardi et al.2005) to simulate the $m_{\rm F160W}$ vs.$m_{\rm F110W}-m_{\rm F160W}$ CMD of Milky-Way stars in a square-degree field of view in the direction of NGC6752. The red crosses overimposed to the diagrams plotted in the left- and middle-panel diagrams of Fig. \[fig:pratio\] are the stars expected in a region with the same area of the analyzed WFC3/NIR field of view 