ΔE 16.1, loose: different enough that you should pick on purpose. Wrack White is much more muted than Greenish White.
Can I use Wrack White instead of Greenish White?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Wrack White is much more muted than Greenish White.
Can I use Greenish White instead of Wrack White?
The same difference applies the other way round: Greenish White is much more saturated.
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.