Summary of the results on the colour selection optimisation, based on the B20 galaxy sample. Top panels: selection completeness (left panel), purity (central panel), and foreground failure rate (right panel), as a function of z_l. The dashed lines represent the selections derived from the full sets of optimal conditions not fitted as a function of z_l, in the case of ground-only (red), Euclid-only (green), and the combination of ground-based and Euclid bands (grey). The solid lines represent the selections obtained from the subsets of optimal conditions, with parameters fitted as a function of z_l, in the case of ground-only (blue) and for the combination of ground-based and Euclid bands (black). Bottom panels: in the left panel, 𝒮 and 𝒮^nf are shown as a function of the iteration number. For the ground-based selection, using griz filters, 𝒮 and 𝒮^nf are represented by solid blue and dashed red lines, respectively. For the selection derived from the combination of ground-based and Euclid filters, namely grizY_EJ_EH_E, 𝒮 and 𝒮^nf are represented by solid black and dashed grey lines, respectively. In the right panel, the difference between and 𝒞 is shown, for the griz (blue lines) and grizY_EJ_EH_E (black lines) selections.