ΔE 8.3, close: similar, with a visible difference side by side. Grey Knights Steel is a little darker than Aluminium.
Can I use Grey Knights Steel instead of Aluminium?
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Grey Knights Steel is a little darker than Aluminium.
Can I use Aluminium instead of Grey Knights Steel?
The same difference applies the other way round: Aluminium 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.