Poison Ivy vs Foul Green

How close are they?

ΔE 30.3, loose: different enough that you should pick on purpose. Foul Green is noticeably lighter, much more saturated, cooler than Poison Ivy.

Can I use Foul Green instead of Poison Ivy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Foul Green is noticeably lighter, much more saturated, cooler than Poison Ivy.

Can I use Poison Ivy instead of Foul Green?

The same difference applies the other way round: Poison Ivy is noticeably darker, much more muted, 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.