in the external region of $\omega\,$Centauri.

As shown in the middle panels of Figure \[fig:stream\], Fe-rich and Fe-poor stellar populations with different nitrogen abundances also populate different RGB regions in the $m_{\rm F814W}$ vs.$C_{\rm F336W,F438W,F814W}$=($m_{\rm F336W}-m_{\rm F438W}$)$-$($m_{\rm F438W}-m_{\rm F814W}$) pseudo-CMD. Specifically, N-poor stars exhibit lower $C_{\rm F336W,F438W,F814W}$ values than N-rich stars with the same luminosity.

The lower panels of Figure \[fig:stream\], show the $I$ vs.$C_{\rm U,B,I}=(U-B)-(B-I)$ pseudo CMD for stars with radial distances larger than $\sim 2.5\,\rm arcmin$, from ground-based photometry. Since the F336W, F438W and F814W filters are the [*HST*]{}-analogous of $U$, $B$ and $I$, the $C_{\rm F336W,F438W,F814W}$ and $C_{\rm U,B,I}$ have similar sensitivity to stellar populations with different chemical composition, we exploited *HST* photometry to disentangle stellar populations with different nitrogen abundances, and then translated the separation into ground-based photometry.

The black solid lines superimposed on the $m_{\rm F814W}$ vs.$C_{\rm F336W,F438W,F814W}$ and the $I$ vs.$C_{\rm U,B,I}$ diagrams are derived as in Section \[sec:selection\] and mark the bluest boundary of the RGBs. We determined the gray lines in the the $m_{\rm F814W}$ vs.$C_{\rm F336W,F438W,F814W}$ diagrams with the criteria to separate most of N-poor stars from N-rich stars, as selected in the ChM shown in the top panel.

To separate 