Upper panel: CCFs measured from the IllustrisTNG300-1 simulation. The black curve shows the full sample. The blue, green, and red curves correspond to the CCFs of the respective colour subsamples. The violet dashed curve shows the CCF for the (B+R) sample selected as explained in Sect. 4. Lower panels: Ratios of the CCFs of the red, green, blue, and violet subsamples to that of the total sample. Shaded regions indicate uncertainties estimated via jackknife resampling.