Colour–magnitude diagram for all galaxies lying in the projected distance of r_500,WL  for XLSSC 27 (at z = 0.295) in the XXL sample (top panel) and an area selected as background within the same projected distance (bottom panel). The dashed line shows the best linear fit for the candidate cluster members, also shown in the bottom panel for reference. The galaxies selected to account for the total K-band luminosity of the cluster and the field are shown in green (K_BCG ≤ K<K^∗ + 3 and ± 0.3 mag from the dashed line), while the BCG is highlighted in red. The colour sequence for the clusters appears more populated (in particular at bright magnitudes) than the background region (field) as expected.