Gun Metal vs Light Metallic Blue

How close are they?

ΔE 31.8, loose: different enough that you should pick on purpose. Light Metallic Blue is a little lighter, much more saturated than Gun Metal.

Can I use Light Metallic Blue instead of Gun Metal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Light Metallic Blue is a little lighter, much more saturated than Gun Metal.

Can I use Gun Metal instead of Light Metallic Blue?

The same difference applies the other way round: Gun Metal 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.