Elysian Green vs Fallen Grass

How close are they?

ΔE 28.2, loose: different enough that you should pick on purpose. Fallen Grass is a little darker, much more muted, cooler than Elysian Green.

Can I use Fallen Grass instead of Elysian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Fallen Grass is a little darker, much more muted, cooler than Elysian Green.

Can I use Elysian Green instead of Fallen Grass?

The same difference applies the other way round: Elysian Green 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.