Dreadful Visage vs Cool Blue

How close are they?

ΔE 30.6, loose: different enough that you should pick on purpose. Cool Blue is noticeably darker, much more saturated, cooler than Dreadful Visage.

Can I use Cool Blue instead of Dreadful Visage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Cool Blue is noticeably darker, much more saturated, cooler than Dreadful Visage.

Can I use Dreadful Visage instead of Cool Blue?

The same difference applies the other way round: Dreadful Visage is noticeably lighter, much more muted, warmer.

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.