Dwarf Shield vs Gunmetal Blue

How close are they?

ΔE 42.1, loose: different enough that you should pick on purpose. Gunmetal Blue is noticeably darker, much more muted, cooler than Dwarf Shield.

Can I use Gunmetal Blue instead of Dwarf Shield?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Gunmetal Blue is noticeably darker, much more muted, cooler than Dwarf Shield.

Can I use Dwarf Shield instead of Gunmetal Blue?

The same difference applies the other way round: Dwarf Shield is noticeably lighter, much more saturated, warmer.

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.