Black Legion vs Basilisk Green

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Basilisk Green is noticeably lighter, more saturated than Black Legion.

Can I use Basilisk Green instead of Black Legion?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Basilisk Green is noticeably lighter, more saturated than Black Legion.

Can I use Black Legion instead of Basilisk Green?

The same difference applies the other way round: Black Legion is noticeably darker, 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.