Location of SDSS galaxies in rest-frame (g − r)–(u − g) colour space. Only galaxies with a solid star forming vs. quenched classification are included in this figure. Density contours are shown as light white lines, indicating that bimodality in ΔSFR is accompanied by bimodality in colour. Additionally, the ((g − r)–(u − g)) plane is subdivided into small hexagonal regions, each colour coded by the fraction of quenched galaxies (as indicated by the colour bar). The optimal linear decision boundary in colour space to classify star forming and quenched systems is shown by a dashed black line. Overlaid on the figure is the purity and completeness obtained by this cut (shown respectively for red, blue objects).