Same X-ray colour-colour diagram as Fig. 10 with information of R(Fe) added by the symbol size (the larger the size, the larger the R(Fe) value), to illustrate the Compton-thick AGN selection by the X-ray colours and R(Fe) alone (without spectral fitting). The error bars of the X-ray colours are omitted for clarity. Sources with strong Fe K lines (EW ≥ 1 keV, measured by spectral fitting) are distinguished by colour. The seven Compton thick AGN candidates are marked by red triangles. Requiring S/M <  1.5 and H/M >  0.5 ensures a hard spectrum of a reflection-dominated source.