Necrotic Flesh vs Hellbound Flesh

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Hellbound Flesh is a little lighter, much more muted than Necrotic Flesh.

Can I use Hellbound Flesh instead of Necrotic Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Hellbound Flesh is a little lighter, much more muted than Necrotic Flesh.

Can I use Necrotic Flesh instead of Hellbound Flesh?

The same difference applies the other way round: Necrotic Flesh is a little darker, much more saturated.

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.