Emerald Green vs Foul Green

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Foul Green is noticeably lighter, more muted, cooler than Emerald Green.

Can I use Foul Green instead of Emerald Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Foul Green is noticeably lighter, more muted, cooler than Emerald Green.

Can I use Emerald Green instead of Foul Green?

The same difference applies the other way round: Emerald Green 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.