H−W2 colour as a function of absolute H-band magnitude, with photometry of late T and Y dwarfs within 20 pc from Kirkpatrick et al. (2019) plotted as black circles. Overplotted in the left panel are isochrones of chemical equilibrium models from this work (in blue), from B03 (in orange), and from SM08 (in green). Overplotted in the right panel are isochrones of chemical equilibrium models (in blue), and non-equilibrium chemistry models with weak and strong vertical mixing (in purple and red, respectively).