Model COSMOS colour tracks and COSMOS sources. The COSMOS filters are different to Euclid and LSST/Pan-STARRS, resulting in slight differences in the tracks presented in Fig. 1. The individual population models are however indicated in the same way as in that figure: the red tracks show MLTs; the early-type elliptical tracks are blue; and the quasar track is green. We additionally plot sources brighter than J = 23 in the COSMOS catalogue (Laigle et al. 2016). Magenta points indicate sources with photometry best fit by an MLT model. The cyan points indicate sources with photometry best fit by an elliptical galaxy model. Upper: zYJ colours. Lower: YJH colours.