Left panels. Flux ratios were calculated for simulated spectra of stars with identical luminosity in the NIRCam F115W band but different chemical compositions. The 2P spectra are helium and nitrogen-enhanced while being carbon and oxygen-depleted compared to the 1P spectra. Light-blue spectra represent stars with the same C, N, O abundances as 1P stars but enriched in helium, while light-red spectra correspond to stars with 1P-like helium content but different C, N, and O abundances. The bottom, middle, and top panels correspond to M-dwarfs, K-dwarfs, and RGB stars, respectively (Milone et al. 2023a). For 1P stars and 2P-like stars with Y=0.246, the effective temperatures, T_eff, and gravities, log(g), are 3762 K, 5901 K, 4746 K and 5.03, 4.50, 1.82, respectively. Helium-rich stars were modeled with T_eff = 3808 K, 6048 K, 4803 K, and log(g)=5.03, 4.48, 1.78, based on Dotter et al. (2008) isochrones. Right panels. Transmission curves of the filters used in this paper. The Euclid filters (top), the F606W and F814W ACS/WFC and F110W F160W IR/WFC3 filters on board HST (middle), and the F150W2 and F322W2 filters of NIRCam/JWST. We superimpose on each panel the flux ratio between 2P and 1P M-dwarf with different helium and C, N, O abundances.