Evolutionary tracks in the Hδ_A+Hγ_A vs. D4000_n and Hδ_A+Hγ_A vs. [MgFe]′ index planes for the BC03_STELIB (solid), BC03_MILES (dotted), and CB19_MILES (dashed) SSPs (see Table 1 for more details). Left, central and right panels are associated to SSPs with different stellar metallicities: sub-solar (Z_⋆ = 0.004), approximately solar (Z_⋆ = 0.02), and super-solar (Z_⋆ = 0.05 for BC03_STELIB and BC03_MILES, Z_⋆ = 0.05 fro CB19_MILES) respectively. The tracks are colour-coded by the age of the SSP, and the points plotted onto the tracks identify ages of 10^8.5 yr, 10⁹ yr, 10^9.5 yr, and 10¹⁰ yr, corresponding to different SSP models, squares for BC03_STELIB, circles for BC03_MILES, and stars for CB19_MILES.