Matt Grey Primer vs Armageddon Dust

How close are they?

ΔE 23.6, loose: different enough that you should pick on purpose. Armageddon Dust is a little darker, much more saturated than Matt Grey Primer.

Can I use Armageddon Dust instead of Matt Grey Primer?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Armageddon Dust is a little darker, much more saturated than Matt Grey Primer.

Can I use Matt Grey Primer instead of Armageddon Dust?

The same difference applies the other way round: Matt Grey Primer is a little lighter, 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.