Dreadful Visage vs Mummy White

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Mummy White is a little darker, much more saturated, warmer than Dreadful Visage.

Can I use Mummy White instead of Dreadful Visage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Mummy White is a little darker, much more saturated, warmer than Dreadful Visage.

Can I use Dreadful Visage instead of Mummy White?

The same difference applies the other way round: Dreadful Visage is a little lighter, much 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.