Hertzsprung-Russell diagrams of absolute Sloan i and ɀ magnitude versus i − ɀ colour and absolute ɀ and J magnitude versus ɀ − J colour. W1738, W2056, and UGPS0722 were plotted directly using their photometry, except that the i-band magnitude of W2056 is derived from its ɀ-band magnitude and the i − ɀ colour is from its spectrum. The SONORA solar-metallicity theoretical model from Marley et al. (2021) of different surface gravities from 3.5 to 5.5 and of effective temperatures from 550 to 250 K with a step of 50 K are plotted. We concluded that the abnormality of the blue i − ɀ colour of the Y dwarfs in the Fig. 11 is caused by its excess flux in the i-band.