ΔE 15.4, loose: different enough that you should pick on purpose. Pale Skin is a little lighter, cooler than Guilliman Flesh. They are different kinds of paint: Citadel Guilliman Flesh is a one-coat contrast-style paint and Two Thin Coats Pale Skin is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Pale Skin is a little lighter, cooler than Guilliman Flesh. They are different kinds of paint.
The same difference applies the other way round: Guilliman Flesh is a little darker, 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 Guilliman Flesh on the colour wheel See Pale Skin on the colour wheel
All matches: Guilliman Flesh equivalents · Pale Skin equivalents