Cold Steel vs Gunmetal Blue

How close are they?

ΔE 25.3, loose: different enough that you should pick on purpose. Gunmetal Blue is noticeably darker, warmer than Cold Steel.

Can I use Gunmetal Blue instead of Cold Steel?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Gunmetal Blue is noticeably darker, warmer than Cold Steel.

Can I use Cold Steel instead of Gunmetal Blue?

The same difference applies the other way round: Cold Steel is noticeably lighter, cooler.

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.