Si and Mg line-strength indices as a function of stellar population age for different IMF shapes and stellar metallicity components. Each curve represents a SSP built using the IMF shapes from Fig. C.1, with line styles and colours matched. Star symbols denote two-component SSP models that include a metal-rich population ([M/H] = +0.26 dex) combined with a varying fraction of a metal-poor component ([M/H] = −1.49 dex). Star sizes increase with the metal-poor fraction: 0%, 5%, 10%, and 20% by mass. We also show the impact of different aperture effects, with the two aperture extractions from massive ETGs shown as filled circles for central (1.2 kpc) apertures and open squares for spectra extracted within 1R_e.