ΔE 21.9, loose: different enough that you should pick on purpose. Plate Armour is noticeably darker than Stormhost Silver.
Can I use Plate Armour instead of Stormhost Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Plate Armour is noticeably darker than Stormhost Silver.
Can I use Stormhost Silver instead of Plate Armour?
The same difference applies the other way round: Stormhost Silver is noticeably 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.