ΔE 7.1, close: similar, with a visible difference side by side. Grey Knights Steel is a little darker than Natural Steel.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Grey Knights Steel is a little darker than Natural Steel.
The same difference applies the other way round: Natural 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.
See Natural Steel on the colour wheel See Grey Knights Steel on the colour wheel
All matches: Natural Steel equivalents · Grey Knights Steel equivalents