Grey Knights Steel vs Bright Gun Metal

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Bright Gun Metal is a little darker than Grey Knights Steel.

Can I use Bright Gun Metal instead of Grey Knights Steel?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Bright Gun Metal is a little darker than Grey Knights Steel.

Can I use Grey Knights Steel instead of Bright Gun Metal?

The same difference applies the other way round: Grey Knights Steel 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.