Brownish Decay vs Militarum Green

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Militarum Green is a little lighter, cooler than Brownish Decay.

Can I use Militarum Green instead of Brownish Decay?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Militarum Green is a little lighter, cooler than Brownish Decay.

Can I use Brownish Decay instead of Militarum Green?

The same difference applies the other way round: Brownish Decay is a little darker, 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.