Gun Metal vs Oily Steel

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Oily Steel is a little darker, more saturated than Gun Metal.

Can I use Oily Steel instead of Gun Metal?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Oily Steel is a little darker, more saturated than Gun Metal.

Can I use Gun Metal instead of Oily Steel?

The same difference applies the other way round: Gun Metal is a little lighter, 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.