Field Green-Grey vs Flat Green

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Flat Green is a little lighter, much more muted, warmer than Field Green-Grey.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Flat Green is a little lighter, much more muted, warmer than Field Green-Grey.

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

The same difference applies the other way round: Field Green-Grey is a little darker, much more saturated, cooler.

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.