Hardness ratios HR2 (top panels) and HR3 (bottom panels) as a function of N_H. In the left panels symbols have been colored according to the spectral type as in Fig. 5 and mean error in the HR is indicated in the lower right corner of the plot. In the right panels, mean HRs have been calculated per bin of N_H and different luminosity classes; different symbols stand for different luminosity classes as indicated in the legend.