Gun Metal vs Dark Metallic Blue

How close are they?

ΔE 36.7, loose: different enough that you should pick on purpose. Dark Metallic Blue is a little darker, much more saturated than Gun Metal.

Can I use Dark Metallic Blue instead of Gun Metal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Dark Metallic Blue is a little darker, much more saturated than Gun Metal.

Can I use Gun Metal instead of Dark Metallic Blue?

The same difference applies the other way round: Gun Metal is a little lighter, 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.