Colour–magnitude diagram of the objects within 20′′ of the cluster centre. Blue galaxies are shown by blue crosses and red galaxies by red squares. Red galaxies with a photometric redshift within 2σ of the peak of the distribution are indicated by filled squares, the bars showing the error in the colour. The greyscale shaded map shows the distribution of BzK-selected galaxies in the GOODS-South field (Daddi et al. 2007a). Colour limits at 3σ are shown by arrows. The typical error in the Y − K_s colour and K_s,   tot magnitude is shown in the bottom left. The solid and dashed lines show the expected colour and magnitude of stellar population synthesis models (Kodama & Arimoto 1997), assuming that star formation begins at z = 10 and z = 3.5, respectively. The composite flux and colour of the “proto-BCG” galaxy assemblage are marked by a green star.