Historically, photometry has been largely used to identify stellar populations (MPs) in Globular Clusters (GCs) by using diagrams that are based on colours and magnitudes that are mostly sensitive to stars with different metallicities or different abundances of helium, carbon, nitrogen and oxygen. In particular, the pseudo two-colour diagram called chromosome map (ChM), allowed the identification and the characterization of MPs in about 70 GCs by using appropriate filters of the Hubble Space Telescope (HST) that are sensitive to the stellar content of He, C, N, O and Fe. We use here high-precision HST photometry from F275W, F280N, F343N, F373N, and F814W images of Omega Centauri to investigate its MPs. We introduce a new ChM whose abscissa and ordinate are mostly sensitive to stellar populations with different magnesium and nitrogen, respectively, in monometallic GCs. This ChM is effective in disentangling the MPs based on their Mg chemical abundances, allowing us to explore, for the first time, possible relations between the production of these elemental species for large samples of stars in GCs. By comparing the colours of the distinct stellar populations with the colours obtained from appropriate synthetic spectra we provide photometric-like estimates of the chemical composition of each population. Our results show that, in addition to first generation (1G) stars, the metal-poor population of Omega Centauri hosts four groups of second-generation stars with different [N/Fe], namely, 2GA--D. 2GA stars share nearly the same [Mg/Fe] as the 1G, whereas 2GB, 2GC and 2GD stars are Mg depleted by ~0.15, ~0.25 and ~0.45 dex, respectively. We provide evidence that the metal-intermediate populations host stars with depleted [Mg/Fe].
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