ΔE 9.1, close: similar, with a visible difference side by side. Elven Skin is a little darker, more muted than Guilliman Flesh. They are different kinds of paint: Citadel Guilliman Flesh is a one-coat contrast-style paint and Two Thin Coats Elven Skin is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Elven Skin is a little darker, more muted than Guilliman Flesh. They are different kinds of paint.
The same difference applies the other way round: Guilliman Flesh is a little lighter, 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 Guilliman Flesh on the colour wheel See Elven Skin on the colour wheel
All matches: Guilliman Flesh equivalents · Elven Skin equivalents