ΔE 6.9, close: similar, with a visible difference side by side. Plate Armour is a little darker than Grey Knights Steel.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Plate Armour is a little darker than Grey Knights Steel.
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.
See Grey Knights Steel on the colour wheel See Plate Armour on the colour wheel
All matches: Grey Knights Steel equivalents · Plate Armour equivalents