Dark Emerald vs Martian Green

How close are they?

ΔE 24.1, loose: different enough that you should pick on purpose. Martian Green is noticeably lighter, cooler than Dark Emerald.

Can I use Martian Green instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Martian Green is noticeably lighter, cooler than Dark Emerald.

Can I use Dark Emerald instead of Martian Green?

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