Dark Green vs Lupercal Green

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Lupercal Green is noticeably lighter, more saturated than Dark Green.

Can I use Lupercal Green instead of Dark Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Lupercal Green is noticeably lighter, more saturated than Dark Green.

Can I use Dark Green instead of Lupercal Green?

The same difference applies the other way round: Dark Green 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.