Colour selection results, obtained from the B20 catalogue, in case of missing z (solid red), i (solid green), r (dashed orange), and g (dashed grey) bands. The blue curves represent the results from the griz and grizY_EJ_EH_E selections reported in Tables A.1 and A.2. Top panels, from left to right: completeness, purity, foreground failure rate, and background density are shown, in the case of ground-only photometry. Middle panels: colour selections from the combination of ground-based and Euclid photometry. The plot structure is analogous to that of top panels. Bottom panels: difference between and 𝒞, for ground-only observations (left panel) and for the combination of ground-based and Euclid photometry (right panel).