ΔE 18.7, loose: different enough that you should pick on purpose. War Master Green is noticeably lighter than Corvus Black.
Can I use War Master Green instead of Corvus Black?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). War Master Green is noticeably lighter than Corvus Black.
Can I use Corvus Black instead of War Master Green?
The same difference applies the other way round: Corvus Black is noticeably darker.
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.