Hardness-intensity diagram (HID, top panel) and color–color diagram (CCD, bottom panel) of 4U 1820–30 from the NICER observations. The blue crosses, orange triangles, and green stars represent the data observed in 2017, 2019, and 2021, respectively. All bursts are removed, and each point represents a segment of 64 s. The HID and CCD of the persistent emission before each X-ray burst are shown as filled black diamonds. For each panel, from left to right, the spectra evolved from the hard island state to the soft banana state.