Dusty Blue vs Space Grey

How close are they?

ΔE 7.1, close: similar, with a visible difference side by side. Space Grey is a little lighter, warmer than Dusty Blue.

Can I use Space Grey instead of Dusty Blue?

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

Can I use Dusty Blue instead of Space Grey?

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