m_F606W versus m_F606W − m_F150W2 CMD for the selected sample of WDs, with stars colour-coded as in panel (a) of Fig. 8. The solid red and blue lines represent fiducials for the two sequences. Dashed lines denote the ±1σ colour uncertainty (from ASs tests) around each fiducial. The horizontal dashed grey line marks the magnitude below which the two sequences can no longer be separated, as the colour difference between the fiducials becomes comparable to the photometric errors.