Colour–magnitude diagram showing the relationship between the rest-frame B − V colour and the absolute magnitude in the B band for AGN in the SXDS (stars) and GWS (crosses) fields having different X-ray-to-optical flux ratio. All sources have redshifts z ≤ 2.0. The sample of AGN is compared with the sample of normal galaxies in the CDF-S field (Wolf et al. 2001, 2004, 2008) represented with contours. Grey scales of the contours are scaled to the data, where the darkest and brightest show the highest and the lowest density of the sources, respectively. The dashed line shows the Melbourne et al. (2007) limit between galaxies belonging to the red sequence (B − V > 0.6) and those in the blue cloud (B − V < 0.6). The bottom right diagram shows the distribution of objects classified as non-AGN (Compton thick and normal galaxies), having very low values of the X/O flux ratio.