Color-intensity evolutionary diagram for M101 ULX-1 using Swift observations (2006–2012), where S and H are the source counts in the two bands: the soft [0.3−1.5 keV] and hard [1.5−10 keV] passbands. Spectral softness (hardness ratio) C and source soft brightness S increase to the right and, finally, the former saturates at high values of S. Larger values of C indicate a softer spectrum, and vice versa. Blue points are related to the decay part of the outburst (see the light curve in Fig. 4).