Upper panel: the S250/S500 vs. S250/S350 colour–colour relation with galaxies coded according to their physical parameters as in Fig. 4. Middle panel: the dependence of the position of galaxies along the S250/S500 vs. S250/S350 colour–colour relation (X_∥) on the different physical parameters. Lower panel: the histogram of the distribution of galaxies along the colour–colour relation (upper) and the relation between the position of galaxies in the direction perpendicular (Y_⊥) and that along (X_∥) the colour–colour relation. The de-projection of the S250/S500 vs. S250/S350 colour–colour relation is done using the measured linear best fit: S250/S500 = 4.78( ± 0.09)  ×  S250/S350 – 4.59( ± 0.19); r = 0.97.