Metallicity index m_1,0 vs. colour and lines of constant [Fe/H] according to metallicity calibrations from the literature. Filled dots represent the likely RGB star sample; open circles are the candidate RGB stars that were rejected based on the metallicity selection in Fig. 3, and serve to illustrate that this metallicity cut does not significantly bias our NGC 2419 sample. The left panel shows the calibration by Hilker (2000) and Adén (2011) as a function of (b − y)₀ colour. The middle right panel shows the semi-empirical calibrations by Calamida et al. (2007) as a function of (v − y)₀ and (u − y)₀, respectively.