ΔE 23.6, loose: different enough that you should pick on purpose. Green Brown is a little darker, more saturated, warmer than Fallen Grass.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Green Brown is a little darker, more saturated, warmer than Fallen Grass.
The same difference applies the other way round: Fallen Grass is a little lighter, more muted, 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 Fallen Grass on the colour wheel See Green Brown on the colour wheel
All matches: Fallen Grass equivalents · Green Brown equivalents