Contour plots showing the optical and near-infrared point sources at b >  60° after the imposed 2σ zero proper-motion cut. In the upper panel, we display the SDSS g − r vs. UKIDSS J − K colours, and in the bottom panel, the WISE W1 − W2 vs. UKIDSS J − K colours of the sources in the parent sample. Our near- to mid-infrared colour criterion is represented by the red dashed line. The optically red targets with u − g >  1 and r − z >  0.5 colours are displayed as blue filled circles, and GQ 1218+0832 is marked with a red square.