Mass–colour (rest-frame (u − r) corrected for extinction) for the field galaxy population (left panel) and galaxies in a group environment (right panel). Dashed lines show the for the mean redshift of the galaxy population, to separate blue and red galaxies. Grey stars represent the blue galaxies that are considered to be in a transition phase (see Sect. 6.3). The contours represent the density distribution of points in the colour–mass plane.