Dark Concrete vs Grey Blue

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Grey Blue is a little lighter, much more saturated than Dark Concrete.

Can I use Grey Blue instead of Dark Concrete?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Grey Blue is a little lighter, much more saturated than Dark Concrete.

Can I use Dark Concrete instead of Grey Blue?

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