ΔE 4.3, very close: most people won’t see a difference side by side. Grey Knights Steel is a little darker than Plate Mail Metal.
Can I use Grey Knights Steel instead of Plate Mail Metal?
Yes, for most jobs. Very close match (ΔE 4). Grey Knights Steel is a little darker than Plate Mail Metal.
Can I use Plate Mail Metal instead of Grey Knights Steel?
The same difference applies the other way round: Plate Mail Metal 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.