ΔE 11.5, fair: the same colour family, but not a straight swap. Undead Flesh is more saturated than Underhive Ash.
Can I use Undead Flesh instead of Underhive Ash?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Undead Flesh is more saturated than Underhive Ash.
Can I use Underhive Ash instead of Undead Flesh?
The same difference applies the other way round: Underhive Ash is more muted.
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.