ΔE 15.2, loose: different enough that you should pick on purpose. Cockpit Color Nakajima is noticeably lighter than Nurgling Green.
Can I use Cockpit Color Nakajima instead of Nurgling Green?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cockpit Color Nakajima is noticeably lighter than Nurgling Green.
Can I use Nurgling Green instead of Cockpit Color Nakajima?
The same difference applies the other way round: Nurgling Green is noticeably darker.
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.