Panels a–c): SPoT standard SSP models compared with SBF measurements of NGC 1316 (full black circle). The data of NGC 4374 (filled square), and NGC 4621 (filled triangle) from Cantiello et al. (2011a) are also shown. Different line styles mark different [Fe/H] contents: dot-dashed (magenta), long-dashed (green), solid (blue) and dashed (drown) refer to [Fe/H] = −0.7, −0.4, 0.0 and + 0.4 dex, respectively. Models in the age-range 3–14 Gyr are shown. The arrow in panel a) indicates the direction of increasing age. Panels d)–n): SPoT models obtained with non-canonical assumptions. In all cases the initial (reference) population has solar metallicity, t ~ 14 Gyr, and is shown with a blue star, while the final composite population is shown with a different colour and connected with a dotted line to the reference model. The symbols for observational data are the same as in panels a)–c). Panels d)–f): A fraction equal to 50% of the total HB-stars is simulated being HHB. For these models three ages are considered (t = 10, 12 and 14 Gyr, increasing ages are marked with larger symbols). Panels g)–i): a young population of t = 30 Myr (red), or t = 100 Myr (magenta) is added to the old solar one. The fraction in mass of old to young stars is reported in the lower panel. Panels l)–n): old SSPs with various [Fe/H] are mixed to the solar one, as labeled. The mass fraction metal-poor to standard SSPs is shown in the lower panel. (See electronic version of the Journal for a colour version of the figure.)