HID of MAXI J1820+070 during its main 2018 outburst, constructed using HR_BAT/MAXI (left) and HR_XRT (right) as hardness ratios. Squares indicate epochs where nearly simultaneous (within one day) fractional rms variability amplitudes and hardness ratio estimates were obtained in the g′ (green) and i′ (red) LCO bands. For clarity, the Al Sadeem time-resolved epochs are not shown. Yellow diamonds mark the epochs of the spectroscopic campaign during the main outburst (Sect. 6 and Appendix D), with each point corresponding to the closest hardness ratio measurement to the spectroscopic observation.