Dusk Red vs Darkoath Flesh

How close are they?

ΔE 21.7, loose: different enough that you should pick on purpose. Darkoath Flesh is noticeably lighter, warmer than Dusk Red.

Can I use Darkoath Flesh instead of Dusk Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Darkoath Flesh is noticeably lighter, warmer than Dusk Red.

Can I use Dusk Red instead of Darkoath Flesh?

The same difference applies the other way round: Dusk Red 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.