Rotten Green vs Undergrowth Green

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Undergrowth Green is a little darker, more saturated, cooler than Rotten Green.

Can I use Undergrowth Green instead of Rotten Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Undergrowth Green is a little darker, more saturated, cooler than Rotten Green.

Can I use Rotten Green instead of Undergrowth Green?

The same difference applies the other way round: Rotten Green is a little lighter, 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.