W1−W2 vs. J−W1 colour–colour diagrams of all metal-poor T dwarf candidates (blue crosses and arrows) in Table A.1 and in Zhang et al. (2023). The Accident is shown as the black cross and arrow. We also include the solar-metallicity M, L, and T sequences from Pan-STARRS as yellow stars, orange squares, and red dots, respectively, along with L subdwarfs (olive squares), four Y0 dwarf, a Y2 dwarf, the Y4 dwarf (purple triangles), and a T9+Y0 system (red dot mixed with purple triangle). Error bars are included for all sources, except for M and L dwarfs. We also show five iso-metallicity curves from the low-metallicity theoretical model LOWZ with parameters log(g) = 5.0, log₁₀K_zz = 2, solar C/O ratio 0.55. From left to right of the five curves, the effective temperatures of the model decrease from 1600 K to 500 K, with a step of 100 K and 50 K above and under 1000 K, respectively.