Grey Knights Steel vs Tarnished Steel

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Tarnished Steel is a little lighter than Grey Knights Steel.

Can I use Tarnished Steel instead of Grey Knights Steel?

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

Can I use Grey Knights Steel instead of Tarnished Steel?

The same difference applies the other way round: Grey Knights Steel is a little darker.

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.