Temporal evolution of the V − R colour index (black dots). The scaled R-band LC is overplotted for comparison (magenta line, the flux increases along +y axis). The horizontal dashed line marks the weighted mean V − R value of (0.339 ± 0.002) mag with a weighted standard deviation of 0.030 mag about the weighted mean. The insert shows the 2019 flare in details – the maximum of the spectral hardness (the minimum of V − R) precedes the flare by about 10 days (see Sect. 3.5 for details).