Left: classes of X-ray emitting sources are shown in colour-colour space. The clouds for QSOs, elliptical galaxies, and Seyfert 2 galaxies are derived from theoretical colour-redshift tracks computed with LePhare and a subset of SED templates used to determine photo-z in this work. Seyfert 2 and elliptical tracks were limited to z < 1, while the QSO cloud is divided into z < 2.5 and 2.5 < z < 4. The black arrow indicates the evolution of a theoretical QSO at z = 0.2 in colour-colour space with increasing extinction. The ticks along this arrow are steps of Δ_e(b−v) = 0.1. The stellar cloud was derived from a 2D histogram of MIST/MESA isochrones assuming [Fe/H] = −2 (Choi et al. 2016; Dotter 2016; Paxton et al. 2018). Right: same classes on top of the binned eFEDS data (CTP_quality ≥ 2), colour-coded according to the average hardness ratio (see text for details).