Comparison of the observed WD CS and theoretical cooling tracks. (a): m_F814W versus m_F814W − m_F150W2 CMD. (b): m_F606W versus m_F606W − m_F814W CMD. (c): m_F150W2 versus m_F150W2 − m_F322W2 CMD. In all panels, red and blue points represent the bWD and rWD populations identified in Fig. 8a, respectively. The solid and dotted red lines show the cooling tracks of 0.46 M_⊙ He-core and CO-core WDs, respectively. The solid blue line shows the cooling track of a 0.54 M_⊙ CO-core WD. In panel (a), selected effective temperatures are indicated along the tracks. In each panel, the horizontal dashed grey line marks the magnitude below which the two sequences can no longer be separated, as their colour difference becomes comparable to the photometric errors.