Colors of comet C/2013 X1 (PANSTARRS). Left panel: color variation of the cometary coma in December 2015 and January 2016. The filled circles correspond to B − V and the open circles to V − R. The values for the Sun colors are represented by the solid and dashed lines, respectively. Right panel: color diagram B − V vs. V − R for comet C/2013 X1. The red star marks the color of the Sun. The blue diamond and green square indicate the mean value of the colors for long-period (Meech et al. 2009) and short-period (Hainaut & Delsanti 2002) comets.