ΔE 15.6, loose: different enough that you should pick on purpose. Cockpit Green (IJN) is much more saturated than Typhon Ash.
Can I use Cockpit Green (IJN) instead of Typhon Ash?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Cockpit Green (IJN) is much more saturated than Typhon Ash.
Can I use Typhon Ash instead of Cockpit Green (IJN)?
The same difference applies the other way round: Typhon Ash is 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.