Undergrowth Green vs Poison Green

How close are they?

ΔE 43.5, loose: different enough that you should pick on purpose. Poison Green is noticeably lighter, much more saturated than Undergrowth Green.

Can I use Poison Green instead of Undergrowth Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 43). Poison Green is noticeably lighter, much more saturated than Undergrowth Green.

Can I use Undergrowth Green instead of Poison Green?

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

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.