r′−3.6 colour as a function of M_3.6 for the S⁴G sample. The black points show the galaxies used to derive a transformation between r′-band and 3.6 μm magnitudes. The solid red line (M_r′ − M_3.6 = 3) denotes the limit where the SDSS r′-band magnitudes were untrustworthy (i.e. M_r′ − M_3.6 colour too red to be physical). The green line denotes the linear fit, with the gradient and intercept annotated in the plot.