ΔE 13.8, fair: the same colour family, but not a straight swap. Green Sky is a little lighter, more saturated, warmer than Fallen Grass.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Green Sky is a little lighter, more saturated, warmer than Fallen Grass.
The same difference applies the other way round: Fallen Grass is a little darker, 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 Sky on the colour wheel
All matches: Fallen Grass equivalents · Green Sky equivalents