ΔE 12.6, fair: the same colour family, but not a straight swap. Sky Type S is a little darker, more muted, cooler than Undead Flesh.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Sky Type S is a little darker, more muted, cooler than Undead Flesh.
The same difference applies the other way round: Undead Flesh is a little lighter, more saturated, 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.
See Undead Flesh on the colour wheel See Sky Type S on the colour wheel
All matches: Undead Flesh equivalents · Sky Type S equivalents