ΔE 18.9, loose: different enough that you should pick on purpose. Putrid Green is a little darker, much more muted, warmer than Undergrowth Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Putrid Green is a little darker, much more muted, warmer than Undergrowth Green.
The same difference applies the other way round: Undergrowth Green is a little lighter, 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.
See Undergrowth Green on the colour wheel See Putrid Green on the colour wheel
All matches: Undergrowth Green equivalents · Putrid Green equivalents