different light-element abundances in $\omega\,$Centauri
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Work based on high-resolution spectroscopy reveals that stellar groups in different metallicity bins of $\omega\,$Centauri host sub populations with different light-element abundances [e.g. @marino2010; @marino2011; @johnson2010].

An efficient tool to disentangle the distinct populations in GCs is provided by the pseudo two-color diagram called chromosome map, which revealed that $\omega\,$Centauri hosts at least 16 sup-populations [ChM, @milone2015; @milone2017]. Based on multi-band [*HST*]{} photometry of $\omega\,$Centauri, @marino2019 identified three main groups of stars that define distinct streams in the ChM. The stars of each stream span similar intervals of \[Fe/H\] but different content of He, C, N, O and Na. Specifically, the upper stream is composed of stars with extreme nitrogen abundances (hereafter N-rich sample), while middle- and the lower-stream stars have low and intermediate \[N/Fe\], respectively and will be called N-poor sample in the following. N-rich and N-poor stars are represented with blue and red points, respectively, in the chromosome map of $\omega\,$Centauri plotted in the upper panel of Figure \[fig:stream\].

Unfortunately, the [*HST*]{} photometry required to build the ChM is only available for stars with radial distances smaller than $\sim$2.5 arcmin. Hence, alternative photometric diagrams are needed to disentangle N-rich and N-poor stars 