Contour plots for the red and blue BCG sub-samples showing the value as a function of the two varying parameters (formation redshift and metallicity). Dashed white curves correspond to the 68% confidence limit. The coloured regions correspond to the 95% confidence interval. The tested metallicity range is Z = 0.1 Z_⊙ to Z = 5 Z_⊙, but here it is restricted to Z = 0.5 Z_⊙ to Z = 3.5 Z_⊙. The tested formation redshift range is z_form = 3 to z_form = 16. The cross displays the location of the best fit parameters.