ΔE 21.2, loose: different enough that you should pick on purpose. Jade Green is much more saturated than IJN Gray Green Nakajima.
Can I use Jade Green instead of IJN Gray Green Nakajima?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Jade Green is much more saturated than IJN Gray Green Nakajima.
Can I use IJN Gray Green Nakajima instead of Jade Green?
The same difference applies the other way round: IJN Gray Green Nakajima 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.