Lupercal Green vs Legion Green

How close are they?

ΔE 16.0, loose: different enough that you should pick on purpose. Legion Green is noticeably lighter, cooler than Lupercal Green.

Can I use Legion Green instead of Lupercal Green?

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

Can I use Lupercal Green instead of Legion Green?

The same difference applies the other way round: Lupercal Green is noticeably darker, warmer.

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.