Band head strengths (see Eq. (2) and Table 5) measured in the BP and RP spectra of known Galactic (MW, orange points, Alksnis et al. 2001), Large Magellenic Cloud (LMC, black points, Kontizas et al. 2001), and Small Magellenic Cloud (SMC, green points, Morgan & Hatzidimitriou 1995) carbon stars. Only targets within 1 arcsec of a Gaia DR3 source_id are taken into account. Upper panels: locus occupied by non-carbon stars represented by the blue shaded area. Middle and lower panels: targets with weaker or non-existing CN features shown with blue points (i.e. they fall in the shaded areas of the upper panels). The pink broken and full lines delimit the domain occupied by 87% and 98% of the carbon stars with strong CN features, respectively.