Color–color diagrams for all three observations, showing hardness ratios calculated for soft, intermediate, and hard X-ray bands. As discussed in the text, the observed tracks are determined by absorption: following the track from the top right corner,representing the persistent flux, to the bottom left corner, showing dips, absorption increases and therefore both colors become harder. During the deepest dips, the softer color becomes softer again, while the hard color does not change. Thisbehavior can be explained by partial covering (Sect. 2.2). The spectrum hardens considerably during thesedips. The colors denote the four different dip stages (see right panel). The black line indicates our polynomial fit to the track; see Sect. 2.3 for details.