Leviathan vs Beetle Green

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Beetle Green is noticeably lighter, more saturated than Leviathan.

Can I use Beetle Green instead of Leviathan?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Beetle Green is noticeably lighter, more saturated than Leviathan.

Can I use Leviathan instead of Beetle Green?

The same difference applies the other way round: Leviathan 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.