Commando Green vs Fallen Grass

How close are they?

ΔE 9.6, close: similar, with a visible difference side by side. Fallen Grass is more muted, cooler than Commando Green.

Can I use Fallen Grass instead of Commando Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Fallen Grass is more muted, cooler than Commando Green.

Can I use Commando Green instead of Fallen Grass?

The same difference applies the other way round: Commando Green is 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.