The evolution of the color indices g − r (upper two panels) and r − i (lower two panels) of interstellar comet 2I/2019 Q4 (Borisov) with time (left two panels) and the heliocentric distance (right two panels). Data points from the two observatories are discriminated by different symbols, as indicated in the legends. Taking the measurement uncertainties into consideration, we see no evidence of color variation of the comet. The time-average values of the color indices are represented by a dashed–dotted horizontal line, with the gray zone denoting the ﹩\pm 1\sigma ﹩ uncertainty region, in each of the panels. The perihelion epoch and distance of the comet are marked by vertical gray dotted lines.