ΔE 25.5, loose: different enough that you should pick on purpose. Monastic Green is noticeably lighter, much more saturated than Black Templar.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Monastic Green is noticeably lighter, much more saturated than Black Templar.
The same difference applies the other way round: Black Templar is noticeably darker, much more muted.
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 Black Templar on the colour wheel See Monastic Green on the colour wheel
All matches: Black Templar equivalents · Monastic Green equivalents