Magnitude difference between a 1P star and a star with the same F115W magnitude and a chemical composition that resembles 2P stars. The azure points correspond to stars with the same chemical composition as 1P stars but enhanced in helium (Y=0.33). The light-red points are derived from spectra that share the same helium content as 1P stars but are enhanced in N and depleted in both C and O. Left, middle, and right panels show spectra with [Fe/H]=−0.75, −1.50, and −2.30 (Milone et al. 2023a; Ziliotto et al. 2023), respectively. The results for RGB, bright-MS stars, and M -dwarfs are shown in the top, middle, and bottom panel respectively. We display in the top-right corner of – each panel the effective temperature and the logarithm of gravity for 1P spectra.