ΔE 11.0, fair: the same colour family, but not a straight swap. Battlefield Brown is a little darker than Midnight Olive.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Battlefield Brown is a little darker than Midnight Olive.
The same difference applies the other way round: Midnight Olive is a little lighter.
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 Midnight Olive on the colour wheel See Battlefield Brown on the colour wheel
All matches: Midnight Olive equivalents · Battlefield Brown equivalents