Comparison of colour and photo-z selections. From left to right: completeness, purity, foreground failure rate, and background density as a function of z_l, obtained from Flagship v2.1. The solid black lines represent the grizY_EJ_EH_E selection. The dashed green lines show the combination, through the ∨ logical operator, of grizY_EJ_EH_E and photo-z selection (Eq. (9)). The dashed red lines represent the photo-z selection, while the dotted black lines represent the selection based on the p(z_g) mode.