Redshift evolution of the mean rest-frame colours of the DS samples from the SIM: clusters IN (black rhombi), clusters OUT (open circles), normal groups (blue triangles), fossil groups (red squares); and from the SAM (green): clusters IN (dashed lines), clusters OUT (dotted lines), and groups (dot-dashed lines). It is also indicated the difference in colour during the redshift interval: , averaged over the environments. Points are slightly offset in x-axis for clarity. This mean colour blueshifted by 0.2 dex gives our first criterion for selecting red galaxies. The synthesis model with Z = Z_⊙, z_f = 5, τ = 3.7 Gyr is shown in the U − R panel as magenta line (see text).