X-ray colour–colour diagram, based on the data obtained from the XMM-Newton EPIC cameras, where s, m and h are the detector-response-corrected photon counts in the rest-frame bands of 3−5 keV, 5−9 keV and 9−20 keV, respectively. The four categories, V, A, M and U and their boundaries are indicated. Our reference heavily obscured AGN, PID 144, is labeled in the diagram. The red dashed-line indicates the evolution track of the X-ray colour when a power-law of Γ = 1.8 is modified by various absorbing colmun. The crosses mark log N_H values 21, 22, 22.5, 23, 23.3, 23.5, 23.7, 23.85 and 24 (cm^-2) from the bottom-right to the upper-left along the track. The X-ray colours estimated for the nearby, heavily obscured AGN, NGC 6240, NGC 4945 and NGC 1068 are also plotted. Note that NGC 4945 has a large value of h/m = 6.8, which is outside of the frame (see text for details of these sources). X-ray spectra of the two sources (PID 84 and 366) with h/m ~ 1.4, located in the M and U intervals, respectively, are described in text (Sect. 3.1).