Colour-magnitude diagram of the simulated galaxies. Black dots show individual galaxies, with grey error bars showing the 68% scatter for different sightlines. The red line shows a running median (with shaded 68% PI) of the m_NB − m_BB colour as a function of NB magnitude for all galaxies when we artificially remove their Lyα emission lines. The selection criteria are shown with blue dashed lines; the magnitude threshold is the same as in Sect. 4.6, while the colour threshold is set to m_NB − m_BB <  +0.85, corresponding roughly to the lower limit of the “no Lyα”. Although no galaxy on average falls inside the selection square, the directional variance results in a probability of a few tens of % of being selected, albeit for the brightest galaxies only.