(g′ − r′) vs. r′ colour–magnitude diagram for Abell 1758 (North and South) for objects classified as galaxies from the μ_max − r′ magnitude relation. The vertical dashed lines indicate the magnitude interval where the colour–magnitude relation was computed. The long oblique dashed line shows the mean colour–magnitude relation; the short oblique dotted lines indicate intervals of ±1σ,2σ and 3σ around the colour–magnitude relation. The interval finally adopted of 1.5σ is shown with full lines. The filled circles show the galaxies with measured redshifts, colour–coded as follows: black: galaxies belonging to the cluster according to their spectroscopic redshifts, cyan and magenta for galaxies with lower and higher redshifts than the cluster, but inside the two circles where the two clusters were extracted, blue and red: galaxies with lower and higher redshifts than the cluster, but outside these two circles.