J − H versus H − K color−color diagram from 1976–2013. The key refers to the time windows of orbital cycles and not to the actual observation dates. Each cycle is displayed with a different color code to visualize the long-term color evolution and the impact of the periastron passages. A cycle here consists of phases 0.05–1.05, 1.05–2.05, etc., to include only one periastron passage per cycle. Open squares of the same color show data points close to periastron (t =  −40 days to t =  + 100 days). The arrow indicates the expected evolution from 1970–2014 based on the pre-1998 data. The data points represented by the black plus symbols are from observations obtained before 1978 with a different detector.