Comparison between the colour–colour distributions of DR14 QSOs (green squares) within our J0410 sample and of the remaining J0410 genuine candidates (blue dots, after removing known interlopers; Sect. 3.3). The two source classes occupy comparable colour regions, suggesting that our selection results might be effectively dominated by z​ ∼ ​2.3 AGN/QSOs. This scenario is also supported by the results of our spectroscopic program (Sect. 3.4).