Colour ratio phase curves for different bands combinations. (a) and (b) were derived by our four datasets, while (c) was derived from Hapke et al. (2012). The lines in (a), (b), and (c) are the best-fit results with a third-order, first-order, and second-order polynomial, respectively. All spectra are vertically offset for clarity. The phase angles of the maximum ratio values are marked by the grey region.