Colour counts: source surface density as a function of the SPIRE colour, S₃₅₀/S₂₅₀ (left), and S₅₀₀/S₃₅₀ (right). Histograms are: red solid line, Planck IN; black line, Planck OUT; green, Planck IN lensed fields only; blue line, z< 1 HLS clusters; and orange dashes, Lockman SWIRE. The Planck IN sources (total and/or lensed sources) show a much higher surface density than other samples, owing mainly to our all-sky search strategy. See Sect. 4.3 for details.