Crusted Sore vs Feral Brown

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Feral Brown is noticeably darker, much more muted, warmer than Crusted Sore.

Can I use Feral Brown instead of Crusted Sore?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Feral Brown is noticeably darker, much more muted, warmer than Crusted Sore.

Can I use Crusted Sore instead of Feral Brown?

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