Dark Concrete vs Dark Grey Blue

How close are they?

ΔE 9.8, close: similar, with a visible difference side by side. Dark Grey Blue is a little darker than Dark Concrete.

Can I use Dark Grey Blue instead of Dark Concrete?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Dark Grey Blue is a little darker than Dark Concrete.

Can I use Dark Concrete instead of Dark Grey Blue?

The same difference applies the other way round: Dark Concrete is a little lighter.

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.