Savage Green vs Field Green-Grey

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Field Green-Grey is noticeably darker, much more muted than Savage Green.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Field Green-Grey is noticeably darker, much more muted than Savage Green.

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

The same difference applies the other way round: Savage Green is noticeably lighter, much more saturated.

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.