Evolution of g − r and g − i colours for 22 SNe in the sample. Host and MW extinction have been removed (Sect. 3). The vertical black dashed line shows the light-curve peak epoch (Sect. 4.1). Lines are drawn between data points to guide the eye. For clarity, colour data points with uncertainties > 0.1 mag are not shown. For comparison, SNe IIn 2005ip, 2006jd (Stritzinger et al. 2012), 2006aa, and 2008fq (Taddia et al. 2013) have their colour curves shown with bold lines.