ΔE 7.0, close: similar, with a visible difference side by side. Stonewall Grey is more muted than Undead Skin.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Stonewall Grey is more muted than Undead Skin.
The same difference applies the other way round: Undead Skin is 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.
See Undead Skin on the colour wheel See Stonewall Grey on the colour wheel
All matches: Undead Skin equivalents · Stonewall Grey equivalents