Dead Magenta vs Flayed One Flesh

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Flayed One Flesh is noticeably darker, more saturated, warmer than Dead Magenta.

Can I use Flayed One Flesh instead of Dead Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Flayed One Flesh is noticeably darker, more saturated, warmer than Dead Magenta.

Can I use Dead Magenta instead of Flayed One Flesh?

The same difference applies the other way round: Dead Magenta is noticeably lighter, more muted, 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.