Wyldwood vs Greasy Black

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Greasy Black is a little darker, much more muted than Wyldwood.

Can I use Greasy Black instead of Wyldwood?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Greasy Black is a little darker, much more muted than Wyldwood.

Can I use Wyldwood instead of Greasy Black?

The same difference applies the other way round: Wyldwood is a little 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.