ΔE 23.3, loose: different enough that you should pick on purpose. Pearl Light Blue is much more saturated than Grey Knights Steel. They are different kinds of paint: Citadel Grey Knights Steel is a metallic and Tamiya Pearl Light Blue is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Pearl Light Blue is much more saturated than Grey Knights Steel. They are different kinds of paint.
The same difference applies the other way round: Grey Knights Steel is 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 Pearl Light Blue on the colour wheel
All matches: Grey Knights Steel equivalents · Pearl Light Blue equivalents