Colour–magnitude diagram for 45 stacked clusters (see text). The black points are all the galaxies in a radius of 1 Mpc around each cluster centre, and with an absolute magnitude M_z′ <  −21.0. The red points correspond to the subsample of these galaxies within  ± 0.1 of the cluster redshift. The black line shows the best fit to the colour − magnitude relation drawn from the red points