Polarimetric colour (PC) of comets over the α range of 0° –35°. All data points except 67P in this study are quoted from the NASA/PDS comet polarimetric archive (Kiselev et al. 2017) and from Bagnulo et al. (2021). Panels a and b present the data in pre-perihelion (180° < ν < 350°, where ν means the true anomaly) and post-perihelion (10° < ν ≤ 180°), respectively. There are no available points around the perihelion (350° ≤ ν ≤ 10°). Red circles of 67P are obtained from the least-square linear fit of 67P dust (Figs. 2–4), and their error bars indicate 1σ uncertainties of the fitting. Black open symbols were derived from the comet data observed by aperture polarimetric observations. Thus, the error bars of their PCs were derived from the error propagation of P_r values measured at discrete wavelength points. ‘Garradd’ and ‘LINEAR’ denote comets C/2009 P1 (Garradd) and C/2000 WM1 (LINEAR), respectively.