Necron Compound vs Martian Green

How close are they?

ΔE 27.6, loose: different enough that you should pick on purpose. Martian Green is noticeably darker, much more saturated than Necron Compound.

Can I use Martian Green instead of Necron Compound?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Martian Green is noticeably darker, much more saturated than Necron Compound.

Can I use Necron Compound instead of Martian Green?

The same difference applies the other way round: Necron Compound is noticeably lighter, much 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.