ΔE 19.6, loose: different enough that you should pick on purpose. Fallen Grass is much more muted, cooler than Death Guard Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Fallen Grass is much more muted, cooler than Death Guard Green.
The same difference applies the other way round: Death Guard Green is 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.
See Death Guard Green on the colour wheel See Fallen Grass on the colour wheel
All matches: Death Guard Green equivalents · Fallen Grass equivalents