Iron Warriors vs Martian Green

How close are they?

ΔE 30.6, loose: different enough that you should pick on purpose. Martian Green is noticeably lighter, much more saturated than Iron Warriors.

Can I use Martian Green instead of Iron Warriors?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Martian Green is noticeably lighter, much more saturated than Iron Warriors.

Can I use Iron Warriors instead of Martian Green?

The same difference applies the other way round: Iron Warriors is noticeably darker, 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.