Martian Green vs Amber Green

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Amber Green is noticeably darker, more muted than Martian Green.

Can I use Amber Green instead of Martian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Amber Green is noticeably darker, more muted than Martian Green.

Can I use Martian Green instead of Amber Green?

The same difference applies the other way round: Martian Green is noticeably lighter, more saturated.

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.