Multi-wavelength light curves, X-ray hardness, and optical/UV colours based on Insight-HXMT, Swift and LCO, AAVSO observations of MAXI J1820+070. (a): Optical, UV, and X-ray light curves. (b): Hardness ratios from Insight-HXMT/LE and Swift/XRT. (c)–(f): Optical/UV colours defined by the magnitude difference between the optical or UV high-frequency and low-frequency bands. Note: (1) The different colours of bars at the top of panel (a) are used to differentiate the states of the main outburst, quasi-quiescent states, and three mini-outbursts. ‘mini1’, ‘mini2’, and ‘mini3’ represent the three mini-outbursts, respectively. The other acronyms are explained in the text. The QQS epochs are defined as periods when the g′-band magnitude is fainter than 17.9. (2) The shaded pink region denotes the period of the optical and X-ray dip events. (3) The interval between the first two vertical dashed red lines at MJD ∼ 58303.5 and 58310.7 denotes the period of the HS-to-SS transition. (4) The last two vertical dashed red lines at MJD ∼ 58380.0 and 58393.0 indicate the period of the SS-to-HS transition.