Colour–colour ratio recovery. Left: S₃₅₃/S₅₄₅. Right: S₅₄₅/S₈₅₇. Top: ratio of recovered over injected colour–colour ratio (Rec./In.) as a function of the 545-GHz excess S/N. This is shown for two choices of the CMB template, i.e., ideal (blue squares) or highly contaminated by extragalactic foregrounds (red crosses). Bottom: recovered versus injected colour–colour ratio per bin of input colour–colour ratio. Again, two cases are shown depending on the quality of the CMB template, ideal (squares) or highly contaminated (crosses). The colour scale provides the average S/N of the 545-GHz excess inside each bin of input colour–colour ratio. The blue dotted lines show S₃₅₃/S₅₄₅< 0.9 and S₅₄₅/S₈₅₇> 0.5, which are the colour criteria adopted for source selection.