Broadsword Silver vs Gunmetal Grey

How close are they?

ΔE 27.2, loose: different enough that you should pick on purpose. Gunmetal Grey is noticeably lighter, more saturated than Broadsword Silver.

Can I use Gunmetal Grey instead of Broadsword Silver?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Gunmetal Grey is noticeably lighter, more saturated than Broadsword Silver.

Can I use Broadsword Silver instead of Gunmetal Grey?

The same difference applies the other way round: Broadsword Silver is noticeably darker, 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.