ΔE 13.2, fair: the same colour family, but not a straight swap. Battle of Marathon is noticeably darker than Monster Brown.
Can I use Battle of Marathon instead of Monster Brown?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Battle of Marathon is noticeably darker than Monster Brown.
Can I use Monster Brown instead of Battle of Marathon?
The same difference applies the other way round: Monster Brown is noticeably 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.