ΔE 15.6, loose: different enough that you should pick on purpose. Pearly Gates is noticeably lighter than Stormhost Silver.
Can I use Pearly Gates instead of Stormhost Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Pearly Gates is noticeably lighter than Stormhost Silver.
Can I use Stormhost Silver instead of Pearly Gates?
The same difference applies the other way round: Stormhost Silver is noticeably darker.
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.