Scatter plots comparing the location of our TNG50 simulated galaxy sample, the V01 and P17 galaxy samples, and the volume-limited RESOLVE galaxy sample. The left and right panel show the g − r colour and the H I mass as a function of K_s-band absolute magnitude, respectively. Colours and absolute magnitudes for the simulated TNG50 galaxies are determined as described in Sect. 3.1. For the V01 sample, g − r colours are calculated from the B − R colours using the conversion formula of Fukugita et al. (2011).