Dark Green Blue vs Space Grey

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Space Grey is a little lighter than Dark Green Blue.

Can I use Space Grey instead of Dark Green Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Space Grey is a little lighter than Dark Green Blue.

Can I use Dark Green Blue instead of Space Grey?

The same difference applies the other way round: Dark Green Blue is a little darker.

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.