ΔE 11.9, fair: the same colour family, but not a straight swap. Hellbound Flesh is noticeably lighter than Combat Fatigues.
Can I use Hellbound Flesh instead of Combat Fatigues?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Hellbound Flesh is noticeably lighter than Combat Fatigues.
Can I use Combat Fatigues instead of Hellbound Flesh?
The same difference applies the other way round: Combat Fatigues is noticeably darker.
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.