X-ray colors for each observation and plotted them into the same CD (Fig. 11). The lines corresponding to some spectral models are included in the plot. The points (filled symbols in Fig. 11) clearly distribute along three separate branches. Classes $\chi_2$ and $\chi_4$ follow a flatter trend than classes $\chi_1$ and $\chi_3$. From Fig. 11, one can see that large movements to the right in the CD are likely caused by hardening of the power law component. However, a simple power law model with the “standard” value of 6$\times 10^{22}$cm$^{-2}$ for the interstellar absorption fits only the $\chi_1$ and $\chi_3$ points. On the basis of the CD, the other points (classes $\chi_2$ and $\chi_4$) can be fitted by a power law model with a lower absorption (2$\times 10^{22}$cm$^{-2}$). An absorption of 2$\times 10^{22}$cm$^{-2}$ is compatible also with the $\chi_1$ and $\chi_3$ points only if an additional high-energy cutoff at $\sim$5 keV is included (see lines in Fig. 11). Detailed spectral fits, which will be presented in a forthcoming paper, indeed indicate that both such a reduced absoption and a high-energy cutoff are present. Notice that 6$\times 10^{22}$cm$^{-2}$ is the standard value adopted for GRS 1915+105, while the lower value 2$\times 