Armageddon Dust vs Matt Varnish

How close are they?

ΔE 41.3, loose: different enough that you should pick on purpose. Matt Varnish is noticeably lighter, much more muted than Armageddon Dust.

Can I use Matt Varnish instead of Armageddon Dust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Matt Varnish is noticeably lighter, much more muted than Armageddon Dust.

Can I use Armageddon Dust instead of Matt Varnish?

The same difference applies the other way round: Armageddon Dust is noticeably darker, much more saturated.

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.