ΔE 21.7, loose: different enough that you should pick on purpose. Chromium Oxide Green Dark is noticeably darker than Nurgling Green.
Can I use Chromium Oxide Green Dark instead of Nurgling Green?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Chromium Oxide Green Dark is noticeably darker than Nurgling Green.
Can I use Nurgling Green instead of Chromium Oxide Green Dark?
The same difference applies the other way round: Nurgling Green is noticeably lighter.
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.