Distribution of sources in the z_P1 − y_P1 vs i_P1 − z_P1 diagram of the full PS1 sample (grey points), high-probability HzQs (black circles), and the candidate HzQs selected for spectroscopic follow-up (red circles) and those with a radio detection (red squares). The traditional colour cut at i − z = 2 is marked using the black dashed line, demonstrating that our method assigns a high HzQ probability to several sources that lie below this selection. We also mark P144+50 (red star) in the colour–colour plot, which was discovered using our method at a redshift of z = 5.66, and with i − z = 1.4 lies below the standard photometric selection employed in other studies.