Skeleton Horde vs Mummy White

How close are they?

ΔE 24.3, loose: different enough that you should pick on purpose. Mummy White is noticeably darker, more muted, warmer than Skeleton Horde.

Can I use Mummy White instead of Skeleton Horde?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Mummy White is noticeably darker, more muted, warmer than Skeleton Horde.

Can I use Skeleton Horde instead of Mummy White?

The same difference applies the other way round: Skeleton Horde is noticeably lighter, more saturated, 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.