ΔE 16.0, loose: different enough that you should pick on purpose. Blue Steel is a little darker, much more saturated than Grey Knights Steel.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blue Steel is a little darker, much more saturated than Grey Knights Steel.
The same difference applies the other way round: Grey Knights Steel is a little lighter, much more muted.
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 Grey Knights Steel on the colour wheel See Blue Steel on the colour wheel
All matches: Grey Knights Steel equivalents · Blue Steel equivalents