Field Green-Grey vs Emerald

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Emerald is noticeably lighter, more muted, cooler than Field Green-Grey.

Can I use Emerald instead of Field Green-Grey?

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

Can I use Field Green-Grey instead of Emerald?

The same difference applies the other way round: Field Green-Grey is noticeably darker, more saturated, warmer.

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.