Necron Compound vs Tarnished Steel

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Tarnished Steel is a little lighter, more muted than Necron Compound.

Can I use Tarnished Steel instead of Necron Compound?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Tarnished Steel is a little lighter, more muted than Necron Compound.

Can I use Necron Compound instead of Tarnished Steel?

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.