Upper panel: the S100/S500 vs. S100/S250 colour–colour relation for galaxies coded according to their physical parameters as in Fig. 4. Middle panel: the dependence of the position of galaxies in the direction perpendicular to the S100/S500 vs. S100/S250 colour–colour relation (Y _⊥ ) on the different physical parameters. Lower panel: the histogram of the distribution of galaxies in the direction perpendicular to the colour–colour relation (upper) and the relation between the position of galaxies along (X_∥) and that in the direction perpendicular (Y_⊥) to the colour–colour relation. The de-projection of the S100/S500 vs. S100/S250 colour–colour relation is done using the measured linear best fit: S100/S500 = 7.37(±0.28)  ×  S100/S250 – 2.23(±0.41); r = 0.92.