Differential BP (top panel) and RP (bottom panel) spectra of Solar [C/Fe]-scaled stars, with different [Fe/H]. Each differential spectrum results from reference spectrum – spectrum, where [Fe/H]_ref = −2.5. The shaded areas correspond to the regions we used for our flux ratios, i.e., the Ca II H & K region (purple), the G-band (magenta), the Hβ (yellow), and the Ca II NIR (green) regions.