Pastel Seafoam vs Mummy White

How close are they?

ΔE 38.3, loose: different enough that you should pick on purpose. Mummy White is noticeably darker, more saturated, warmer than Pastel Seafoam.

Can I use Mummy White instead of Pastel Seafoam?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Mummy White is noticeably darker, more saturated, warmer than Pastel Seafoam.

Can I use Pastel Seafoam instead of Mummy White?

The same difference applies the other way round: Pastel Seafoam is noticeably lighter, more muted, cooler.

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.