Skull Linen vs Mummy White

How close are they?

ΔE 20.2, loose: different enough that you should pick on purpose. Mummy White is a little darker, much more saturated than Skull Linen.

Can I use Mummy White instead of Skull Linen?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Mummy White is a little darker, much more saturated than Skull Linen.

Can I use Skull Linen instead of Mummy White?

The same difference applies the other way round: Skull Linen is a little lighter, much more muted.

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.