Necron Compound vs Magnesium

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Magnesium is a little lighter, more muted than Necron Compound.

Can I use Magnesium instead of Necron Compound?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Magnesium is a little lighter, more muted than Necron Compound.

Can I use Necron Compound instead of Magnesium?

The same difference applies the other way round: Necron Compound is a little darker, 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.