Colour curves corrected for host-galaxy extinction and afterglow-subtracted. Filled circles represent detections in both filters and arrows are upper limits derived from detections in one of the two bands. Solid lines represent the colour evolution of SN 1998bw derived from the light-curve fits in Sect. 4.3. For clarity, error bars do not include the systematic error in A_V,host and for the last epoch, upper limits are not shown.