Gun Metal vs Necron Compound

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Necron Compound is a little darker than Gun Metal.

Can I use Necron Compound instead of Gun Metal?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Necron Compound is a little darker than Gun Metal.

Can I use Gun Metal instead of Necron Compound?

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

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.