Randomly selected subsample of 12 galaxies of our extended sample (limited to a redshift lower than 0.2 for our main analysis) with a redshift higher than 0.2. All of them were classified to be elliptical galaxies by our selection criteria, which is confirmed by their morphology. Note that for some of the sources with the smallest angular extent, the SDSS colour-composite image becomes imprecise, causing the apparent rainbow-like colour structure.