Histograms of the optical rest-frame (g − r) colours for the various red sequence selections, colour coded as before. The final NUV + WISE sample has a significantly narrower range compared to the original sequence, but simply choosing galaxies redder than, say, 0.72 would still include potential weak star formers (pink and red histograms) in addition to the members of our ‘best’ sample (dark red histogram with black outlines). Photometric errors are around 0.03 mag.