Colour-colour diagram in the GROND-intrinsic i′z′JHK_s bands of the prime quasars (in blue; triangles denote limits) compared to normal foreground stars and galaxies (in grey) of the same fields, with selected brown dwarfs from earlier GROND observations (in green and purple, and labelled with their spectral types). Our 26 candidates are shown in red; the nine objects which have ALLWISE detections are in light red. The yellow-shaded region indicates the search region for the QSO pair. The arrow at the top right of each panel shows the amount by which a correction for A_V = 1 would shift an object. Except for the grey objects, all are foreground A_V-corrected.