Colour evolution of stellar populations up to 100 Myr according to different models and ingredients. Fixed parameters are indicated at the top while variable ones at the bottom. a) Comparison between SSPs built with Padova tracks and SSPs from Maraston (2005) for different metallicities. b) SSPs with Padova tracks including nebular emission compared to those without nebular emission. c) Same as in b) for CSPs. d) CSP models built with a Kroupa IMF and variable upper stellar mass limit. e) CSP models built with different metallicities. f) CSP models built with different slopes for the IMF above 0.5 M_⊙. Age (Myr) is indicated at selected evolutionary stages in panels b) and c).