the bulk of N-rich and N-poor stars in the ground-based CMD we first estimated the $C_{\rm F336W,F438W,F814W}$ distance between the gray and black fiducials of each CMD for stars with different luminosities ($\Delta  C_{\rm F336W,F438W,F814W}$). We then determined the bluest RGB boundaries in the $I$ vs.$C_{\rm U,B,I}$ for both Fe-poor and Fe-rich stars (lower panels). Finally, we shifted these fiducial lines by a certain amount, $\Delta C_{\rm U,B,I}$ that corresponds to the $\Delta  C_{\rm F336W,F438W,F814W}$ quantity derived from *HST* photometry.\
The selected N-poor and N-rich stars are shown with red and blue circles and triangles, respectively, for both metal-poor and metal-rich stars.

$\omega\,$Centauri hosts a sample of metal-rich stars (\[Fe/H\]$\gtrsim -0.9$) that are called population-$a$ stars and define a distinct RGB sequence, on the red side of the bulk of RGB stars [e.g. @lee1999; @pancino2000]. In the left panels of Figure \[fig:cmds\] we identified population-$a$ stars in the $I$ vs.$U-I$ (lower-left panel) and $I$ vs.$U-I$ CMDs (lower-right panel). The position of population-$a$ stars in the ChM (see upper panel of Figure \[fig:stream\]) corroborate previous conclusion by @marino2011 [@marino2019] that the majority of population-$a$ stars belong to the N-rich sample, and a small group of population-$a$ stars are N-poor. Specifically, $\sim 