Witch Brew vs Moldy Skin

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Moldy Skin is noticeably lighter, warmer than Witch Brew.

Can I use Moldy Skin instead of Witch Brew?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Moldy Skin is noticeably lighter, warmer than Witch Brew.

Can I use Witch Brew instead of Moldy Skin?

The same difference applies the other way round: Witch Brew is noticeably darker, 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.