Hardness-intensity diagram of NICER observations of 4U 1636–53 (left), XTE J1739–285 (middle), and MAXI J1816–195 (right). The hardness ratio is defined as the count ratio of the 4–10 and 0.5–4 keV, and the intensity is defined as the counts in 0.5–10 keV. Each datapoint in the plots corresponds to a 64-second NICER data segment, with the NICER observations analyzed in this work shown in red and other NICER observations marked in gray.