Wyldwood vs Hospitallier Black

How close are they?

ΔE 25.2, loose: different enough that you should pick on purpose. Hospitallier Black is noticeably darker, much more muted than Wyldwood.

Can I use Hospitallier Black instead of Wyldwood?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Hospitallier Black is noticeably darker, much more muted than Wyldwood.

Can I use Wyldwood instead of Hospitallier Black?

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

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.