Field Green-Grey vs Dark Green

How close are they?

ΔE 9.2, close: similar, with a visible difference side by side. Dark Green is a little lighter than Field Green-Grey.

Can I use Dark Green instead of Field Green-Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Dark Green is a little lighter than Field Green-Grey.

Can I use Field Green-Grey instead of Dark Green?

The same difference applies the other way round: Field Green-Grey is a little 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.