Martian Green vs Beetle Green

How close are they?

ΔE 18.5, loose: different enough that you should pick on purpose. Beetle Green is a little darker, cooler than Martian Green.

Can I use Beetle Green instead of Martian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Beetle Green is a little darker, cooler than Martian Green.

Can I use Martian Green instead of Beetle Green?

The same difference applies the other way round: Martian Green is a little lighter, 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.