Feral Green vs Fallen Grass

How close are they?

ΔE 26.7, loose: different enough that you should pick on purpose. Fallen Grass is noticeably darker, much more muted than Feral Green.

Can I use Fallen Grass instead of Feral Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Fallen Grass is noticeably darker, much more muted than Feral Green.

Can I use Feral Green instead of Fallen Grass?

The same difference applies the other way round: Feral Green is noticeably lighter, much more saturated.

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.