ΔE 12.0, fair: the same colour family, but not a straight swap. Underbelly Blue is a little darker than Wrack White.
Can I use Underbelly Blue instead of Wrack White?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Underbelly Blue is a little darker than Wrack White.
Can I use Wrack White instead of Underbelly Blue?
The same difference applies the other way round: Wrack White is a little 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.