ΔE 59.8, loose: different enough that you should pick on purpose. Heavy Armour is noticeably darker than Stormhost Silver.
Can I use Heavy Armour instead of Stormhost Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 60). Heavy Armour is noticeably darker than Stormhost Silver.
Can I use Stormhost Silver instead of Heavy 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.