Armageddon Dust vs Burnt Umber

How close are they?

ΔE 32.0, loose: different enough that you should pick on purpose. Burnt Umber is noticeably darker, warmer than Armageddon Dust.

Can I use Burnt Umber instead of Armageddon Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Burnt Umber is noticeably darker, warmer than Armageddon Dust.

Can I use Armageddon Dust instead of Burnt Umber?

The same difference applies the other way round: Armageddon Dust is noticeably lighter, 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.