ΔE 14.7, fair: the same colour family, but not a straight swap. Boreal Green is noticeably darker, cooler than Medieval Forest.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Boreal Green is noticeably darker, cooler than Medieval Forest.
The same difference applies the other way round: Medieval Forest is noticeably lighter, 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.
See Medieval Forest on the colour wheel See Boreal Green on the colour wheel
All matches: Medieval Forest equivalents · Boreal Green equivalents