Left panels: fraction of red BCGs classified according to their ^0.1(u − r) colour; centre left panels: fraction of early-type BCGs classified according to their concentration parameter; centre right panels: fraction of BCGs classified simultaneously as red and early-type from their colour and concentration parameter; right panels: fraction of BCGs classified as elliptical from their Galaxy Zoo morphology. All fractions are shown as a function of absolute magnitude. Top panels compare compact groups with loose groups of low and high mass, while bottom panels compare compact and loose groups with similar total absolute magnitude. Vertical error-bars are obtained by using the bootstrap resampling technique. Horizontal error-bars are the 25% and 75% quartiles of the absolute magnitude distribution within each bin.