R_C − K colour versus K-band magnitude of X-ray AGN in the Lockman Hole. The blue and red symbols are identical to Figs. 4 and 5, while the grey dots represent the colours of the optical-UKIDSS normal galaxies. The left-side and right-side coloured histograms refer to the open and filled symbols, while the grey histogram on both sides refers to the grey dots. This figure is available in color in electronic form.