Martian Ironearth vs Red Iron Oxide

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Red Iron Oxide is much more saturated, cooler than Martian Ironearth.

Can I use Red Iron Oxide instead of Martian Ironearth?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Red Iron Oxide is much more saturated, cooler than Martian Ironearth.

Can I use Martian Ironearth instead of Red Iron Oxide?

The same difference applies the other way round: Martian Ironearth is much more muted, 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.