Holy White vs Dreadful Visage

How close are they?

ΔE 19.9, loose: different enough that you should pick on purpose. Dreadful Visage is noticeably darker, more saturated than Holy White.

Can I use Dreadful Visage instead of Holy White?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Dreadful Visage is noticeably darker, more saturated than Holy White.

Can I use Holy White instead of Dreadful Visage?

The same difference applies the other way round: Holy White is noticeably lighter, 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.