fitted with a model consisting of the sum of a power-law and a multicolor disk-blackbody, modified by interstellar absorption (see e.g. Belloni et al. 1997b). This model has become standard for the fitting of spectra of black-hole candidates. Figure 1 shows a CD with the location of the two models for different values of their parameters (photon index $\Gamma$ and temperature at the inner radius kT$_{in}$). The interstellar absorption has been fixed to $6\times 10^{22}$cm$^{-2}$, as determined from spectral fitting ([@bel97b; @mar99]). With our choice of colors, HR$_2$ is mostly sensitive to changes in the slope of the power law component and HR$_1$ to changes in the temperature of the disk component for the ranges shown in Fig. 1. The additional presence of an iron line or edge, although significant for the $\chi^2$ fitting of energy spectra, does not modify the colors much compared to the overall range covered by the source. It is important to note that the color analysis is by itself independent of a specific model. Therefore, although we interpret the results in the framework of a power law plus disk model, all the observed color variations are model independent.

In addition to CDs, we also produced 