(a–b) Magnetic flux evolution and color curves for active region NOAA 12118. The vertical dashed lines mark the transitions between different evolutionary stages (see text). (c–d) Magnetic flux evolution and color curves for active region NOAA 12673. Vertical lines indicate flares and thick vertical lines indicate X-class flares. The time is in decimal days measured from the beginning of their observation, that is, at 17 July 2014 16:34 UT for NOAA 12118 and at 28 August 2017 08:58 UT for NOAA 12673.