ΔE 17.4, loose: different enough that you should pick on purpose. Flat Green is a little darker, much more saturated than Typhon Ash.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Flat Green is a little darker, much more saturated than Typhon Ash.
The same difference applies the other way round: Typhon Ash is a little lighter, much more muted.
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 Typhon Ash on the colour wheel See Flat Green on the colour wheel
All matches: Typhon Ash equivalents · Flat Green equivalents