ΔE 21.2, loose: different enough that you should pick on purpose. Pearly Gates is noticeably lighter than Runefang Steel.
Can I use Pearly Gates instead of Runefang Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Pearly Gates is noticeably lighter than Runefang Steel.
Can I use Runefang Steel instead of Pearly Gates?
The same difference applies the other way round: Runefang Steel 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.