Stellar mass versus g − i colour relations for galaxies in the Virgo (top) and Fornax (bottom) clusters (for other colours, see Fig. C.1). The solid black lines show the running mean relations (in M_⋆ with a bin width of 1 dex) for the ETGs (bold symbols), while the dashed lines indicate the mean minus one σ. The Virgo relations are shown (in grey) in the lower panel for reference. The relations are largely consistent for the Fornax and Virgo clusters.