Left column: The grz − (g − r) plane for the white dwarf stars in from the Gaia EDR3 catalogue (Gentile Fusillo et al. 2021). The black rectangle shows the region of interest in our study. Right column: The grz histogram for objects inside the black rectangle. The white dwarf and non-white dwarf objects histograms are shown as the light and dark blue histograms, respectively. The solid blue line corresponds to their sum. The solid red line marks grz = 0, the centre of the BHB distribution.