Application of the calibrated colour selections to the Flagship simulated sample described in Sect. 4.4. From left to right: completeness, purity, foreground failure rate, and background density as a function of z_l, from the fitted colour selections based on griz and grizY_EJ_EH_E bands, adopting 5σ magnitude limits. The assumed z_l precision is δz_l = 10⁻³. The solid blue and solid black lines represent the griz and grizY_EJ_EH_E selections, respectively, applied to the B20 catalogue. The dashed red and dashed green curves represent the griz and grizY_EJ_EH_E selections, respectively, applied to the Flagship v2.1 catalogue (Euclid Collaboration, in prep.).