Martian Ironcrust vs Burnt Umber

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Burnt Umber is noticeably darker, much more muted, warmer than Martian Ironcrust.

Can I use Burnt Umber instead of Martian Ironcrust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Burnt Umber is noticeably darker, much more muted, warmer than Martian Ironcrust.

Can I use Martian Ironcrust instead of Burnt Umber?

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

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.