Colour-colour diagrams of the zero proper motion sources. The quasars observed in this work are shown by the red filled circles, the previously known quasars by the grey squares, and the stellar sources by the yellow star symbols. Limits are denoted by triangles, which are colour-coded in the same way. Left panel: u − g vs. g − r colours of the targets, which represent the UVX selection, middle panel: g − J vs. J − K colours (KX selection), right panel: WISE colours W1 − W2 vs. W2 − W3. For comparison, typical stellar colours from the catalogue of Hewett et al. (2006) are shown as the dark grey squares in the left and middle panel. Middle panel: selection line from Maddox et al. (2008, 2012), selecting targets with a K-band excess relative to typical stellar colours. Right panel: selection criterion of W1 − W2 >  0.8 presented by Stern et al. (2012) as the dashed line and the selection wedge defined by Mateos et al. (2012) as the dotted line.