Colour–colour diagram of the 2151 Planck high-z source candidates (red dots), and other typical astrophysical sources. Galactic cold clumps from the PGCC catalogue are shown as grey dots. Triangles represent the nine DSFGs discovered by SPT, with confirmed redshifts from 2.5 to 4.5. The square is the “Cosmic Eyelash” submm galaxy, lying at z = 2.33. The colour of the symbols is proportional to the redshift of the object from 2 to 6 (from green to blue). Contours give the pixel distribution of the full mask (orange) and towards cirrus (purple), including 99.9%, 50%, and 10% of the distribution. Notice that the colours of PHZ sources are computed using flux densities obtained on cleaned maps, , , and . The dashed lines show the two criteria used to build the colour–colour selection.