Optical/X-ray light curves, hardness ratio, and optical colour evolution during the dip events. (a) X-ray light curves from Insight-HXMT; (b) g′, i′, V-band light curves and 3σ confidence curve of Celerite2 for the V-band light curve; (c) HXMT hardness; (d) g′−i′ colour. The pink area represents the epoch of optical and X-ray dips. The interval between the two vertical dashed red lines marks the first IMS period.