U−R colour distribution of galaxies, along with age–colour and metallicity–colour relations, for galaxies in the cluster cores (first row), cluster outskirts (second row), normal groups (third row) and FGs (fourth row) at different epochs: shown, respectively, in blue/red are objects with SSFR during the last Gyr higher/lower than a value equivalent to 1 M_⊙ yr⁻¹ for a 10¹⁰M_⊙ galaxy.