Martian Ironcrust vs Red Iron Oxide

How close are they?

ΔE 18.1, loose: different enough that you should pick on purpose. Red Iron Oxide is a little lighter, much more saturated than Martian Ironcrust.

Can I use Red Iron Oxide instead of Martian Ironcrust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Red Iron Oxide is a little lighter, much more saturated than Martian Ironcrust.

Can I use Martian Ironcrust instead of Red Iron Oxide?

The same difference applies the other way round: Martian Ironcrust is a little 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.