ΔE 13.6, fair: the same colour family, but not a straight swap. Sky Grey is noticeably darker than Wrack White.
Can I use Sky Grey instead of Wrack White?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Sky Grey is noticeably darker than Wrack White.
Can I use Wrack White instead of Sky Grey?
The same difference applies the other way round: Wrack White 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.