g − r versus r − i colour distribution of simulated magnitude sources from the combined catalogue in the COSMOS2020 field used to optimise the selection function of companions to W0410–0913 in the redshift range 3.531 < z < 3.731. The left panel represents galaxies at lower redshifts (z < 3.531, grey) and higher redshifts (z > 3.731, tan). The middle panel displays stars, X-ray sources, and sources with failed photometric redshift measurements, while the right panel shows galaxies in the targeted redshift range of 3.631 ± 0.1. The solid magenta line represents the optimised selection function at 3.531 < z < 3.731. The grey line shows the colour–redshift track of the LBG composite spectrum of Shapley et al. (2003) with the IGM absorption of Madau (1995) shifted from z = 3.0 to z = 4.25, where the dots indicate Δz = 0.25. We show the representative colours of several classes of galaxies from Coleman et al. (1980) as a function of redshift from z = 0 to z = 3.0.