Dusty Blue vs Viking Grey

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Viking Grey is a little darker, warmer than Dusty Blue.

Can I use Viking Grey instead of Dusty Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Viking Grey is a little darker, warmer than Dusty Blue.

Can I use Dusty Blue instead of Viking Grey?

The same difference applies the other way round: Dusty Blue is a little 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.