ΔE 17.4, loose: different enough that you should pick on purpose. Barley Gray BS4800/18B21 is noticeably darker, much more muted than Toxic Mist.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Barley Gray BS4800/18B21 is noticeably darker, much more muted than Toxic Mist.
The same difference applies the other way round: Toxic Mist is noticeably lighter, much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Toxic Mist on the colour wheel See Barley Gray BS4800/18B21 on the colour wheel
All matches: Toxic Mist equivalents · Barley Gray BS4800/18B21 equivalents