∆_{CF275W,F336W,F438W} vs. ∆_F275W,F277W ChM for bright MS stars (top) and ∆_F115W,F277W vs. ∆_F475W,F814W for M dwarfs (bottom). The insets show the corresponding Hess diagrams. The arrows show the effect of changing, one at time, the abundances of helium mass fraction Y, [N/Fe], [O/Fe], and [Fe/H] by 0.05, 1.2, −0.5, and 0.3 dex with respect to a reference stellar population with [Fe/H]=−1.7, [α/Fe]=0.4 and solar carbon and nitrogen contents (see Milone et al. 2018b; Marino et al. 2019, for details).