Color–color diagram of all PCU2 measurements of 4U 1820-30 between March 31, 1999, and March 31, 2011. Colors are defined as in Homan et al. (2010): soft color is the count rate in PCU2 channels 10−17 to 6−9 (corresponding to ranges 4.1−7.4 and 2.5−4.1 keV) and hard color is channels 24−40 to 18−23 (17.1−9.9 and 7.4−9.9 keV). Each point is for 128 s of data. Burst data have been excluded. The stars refer to the colors of the 128-s data stretches immediately before the 4 superexpansion bursts, the open circles to those of the other 5 bursts in the low state, squares to all other ordinary bursts and the large cross to the superburst. One ordinary burst is not included here. The burst recurrence time is 2.5 times larger at the group of circles with coordinates (1.95, 1.09) than at the group of stars with coordinates (2.05, 1.25). (This figure is available in color in the electronic form.)