Colours with respect to spectral type for cool and ultracool dwarfs. The adopted spectral type of W1738 is YO (Cushing et al. 2011) and that of W1828 is Y2 (Cushing et al. 2021). Upper panel: i − ɀ colour of W1738 obtained by GTC/HiPERCAM photometry in 2021 compared with dwarfs with photometry from Best et al. (2017); Leggett et al. (2012), a T9 dwarf UGPSO722 with photometry from Leggett et al. (2013), and a Y0 dwarf W2056 with systhesised i − ɀ colour using a spectrum from Leggett et al. (2013). Lower panel: ɀ − J colour of W1738 compared with dwarfs with photometry from Lodieu et al. (2013), Best et al. (2017), the ɀ magnitude of W2056 (Lodieu et al. 2013), the J magnitude of W1828, W2056 (Leggett et al. 2013), and UGPS0722 (Tinney et al. 2018). The photometry systems were under the PS1 AB system and the 2MASS system. The photometric errors are smaller than the data points.