Far infrared colour–colour diagrams, equivalent to those shown in Fig. 2, with galaxies coded according to their effective surface brightness, with grey open triangles for objects with μ_e(H) > 19 mag arcsec^-2 and yellow asterisks for galaxies with μ_e(H) ≤ 19 mag arcsec^-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 H band effective surface brightness.