ΔE 17.8, loose: different enough that you should pick on purpose. Interior Grey Green is noticeably lighter, more muted than Fallen Grass.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Interior Grey Green is noticeably lighter, more muted than Fallen Grass.
The same difference applies the other way round: Fallen Grass is noticeably darker, 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.
See Fallen Grass on the colour wheel See Interior Grey Green on the colour wheel
All matches: Fallen Grass equivalents · Interior Grey Green equivalents