Gunmetal Grey vs Ultramarine Blue

How close are they?

ΔE 18.1, loose: different enough that you should pick on purpose. Ultramarine Blue is a little darker, much more saturated than Gunmetal Grey.

Can I use Ultramarine Blue instead of Gunmetal Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ultramarine Blue is a little darker, much more saturated than Gunmetal Grey.

Can I use Gunmetal Grey instead of Ultramarine Blue?

The same difference applies the other way round: Gunmetal Grey is a little lighter, 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.