ΔE 15.4, loose: different enough that you should pick on purpose. Cockpit Green (IJN) is a little darker, more saturated, cooler than Underhive Ash.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cockpit Green (IJN) is a little darker, more saturated, cooler than Underhive Ash.
The same difference applies the other way round: Underhive Ash is a little lighter, more muted, warmer.
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 Underhive Ash on the colour wheel See Cockpit Green (IJN) on the colour wheel
All matches: Underhive Ash equivalents · Cockpit Green (IJN) equivalents