J0524: OGLE V − I colour index vs. I (interpolated linearly to the times of the V-band measurements). Left panel: we plot the complete data set. Right panel: we restrict the times to MJD 55400–55600 (see shaded region in Fig. 20). Apart from a general trend of redder when brighter, smoothing the date reveals a hysteresis loop (see right panel), where the rise of a flare is bluer than the decay.