Martian Ironearth vs Raw Sienna

How close are they?

ΔE 25.3, loose: different enough that you should pick on purpose. Raw Sienna is noticeably lighter, much more saturated than Martian Ironearth.

Can I use Raw Sienna instead of Martian Ironearth?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Raw Sienna is noticeably lighter, much more saturated than Martian Ironearth.

Can I use Martian Ironearth instead of Raw Sienna?

The same difference applies the other way round: Martian Ironearth 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.