Rainforest vs Elven Green

How close are they?

ΔE 27.4, loose: different enough that you should pick on purpose. Elven Green is a little darker, much more muted, cooler than Rainforest.

Can I use Elven Green instead of Rainforest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Elven Green is a little darker, much more muted, cooler than Rainforest.

Can I use Rainforest instead of Elven Green?

The same difference applies the other way round: Rainforest is a little lighter, much 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.