Colour-magnitude diagram for a single field showing all the simulated sources in black and those recovered as PNe in red (see Sect. 3.6). The 99% and 99.99% limits for the continuum sources are shown in blue, while the region beyond the limiting magnitude is shown in grey. The dotted black line shows the colour selection adopted to limit contamination from background galaxies. The histograms (in logarithmic scale) show the number of PNe recovered as a function of colour (right) and magnitude (top) with the simulated sources in black and those recovered as PNe in red.