ΔE 31.2, loose: different enough that you should pick on purpose. Skull Linen is a little darker, much more muted than Guilliman Flesh.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Skull Linen is a little darker, much more muted than Guilliman Flesh.
The same difference applies the other way round: Guilliman Flesh is a little lighter, much 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 Skull Linen on the colour wheel
All matches: Guilliman Flesh equivalents · Skull Linen equivalents