Flayed One Flesh vs Buff

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Buff is a little darker, cooler than Flayed One Flesh.

Can I use Buff instead of Flayed One Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Buff is a little darker, cooler than Flayed One Flesh.

Can I use Flayed One Flesh instead of Buff?

The same difference applies the other way round: Flayed One Flesh is a little lighter, warmer.

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.