Black Metal vs Gunmetal

How close are they?

ΔE 22.2, loose: different enough that you should pick on purpose. Gunmetal is noticeably darker, more muted than Black Metal.

Can I use Gunmetal instead of Black Metal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Gunmetal is noticeably darker, more muted than Black Metal.

Can I use Black Metal instead of Gunmetal?

The same difference applies the other way round: Black Metal is noticeably lighter, more saturated.

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.