Opal Skin vs Wrack White

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Wrack White is a little lighter, much more muted than Opal Skin.

Can I use Wrack White instead of Opal Skin?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Wrack White is a little lighter, much more muted than Opal Skin.

Can I use Opal Skin instead of Wrack White?

The same difference applies the other way round: Opal Skin is a little darker, 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.