X-ray colour − colour diagram of the Lockman Hole sources. EROs (with R_C − K > 5) are plotted with black symbols, while the non-ERO population is plotted in grey. The symbols (open-filled circles and asterisks) are identical to Fig. 4. The horizontal line at HR1 =  −0.4 is the dividing line between soft and hard sources adopted for this paper and the purple region is the region where the “leaky absorber” effect is important (see text). The cross in the top right corner shows a typical error-bar with dHR1 = 0.15 and dHR2 = 0.20, while crosses on the individual sources mark AGN with a known spectroscopic redshift.