Predicted dependence of PS1 i − ɀ colours with spectral type of late-T and Y dwarfs using the observational spectra of T dwarf standards from Burgasser et al. (2000, 2003a); Kirkpatrick et al. (2008); T subdwarfs from Lodieu et al. (2022); Zhang et al. (2023); UGPSO722 and W2056 (Leggett et al. 2013, 2021); and the theoretical spectra of Morley et al. (2014), respectively. We adapted the spectral type–effective temperature relation using Table 1 in Martín et al. (2021). Three curves represent different surface gravities of the model, with fixed sedimentation efficiency and cloud cover. The i − ɀ colours of the Y dwarfs (W1738 and W2056) are bluer than these models for all the available choices of physical parameters. The mid-T dwarf 2MASS J07554795+2212169 and W1738 in 2014 were contaminated by two blue galaxies.