Colour-magnitude diagram of MAXI J1820+070 showing the correlation between the g′-band magnitude and the g′−i′ colour. The horizontal line represents the threshold of quasi-quiescence (i.e. fainter than 17.9). The vertical line indicates the division of optical colour between the QQS period and the periods of the main outburst and three mini-outbursts. Data points in this figure are colour-coded according to the states defined in Fig. 1.