Far infrared colour–colour diagrams, equivalent to those shown in Fig. 2, with galaxies coded according to their birthrate parameter, with cyan open squares for objects with log b > 0.2 and magenta three points stars for galaxies with log b ≤ 0.2. The black histogram gives the distribution of all galaxies along the X-axis, while the coloured histograms those of the two subsamples of galaxies selected according to their birthrate parameter.