ΔE 14.2, fair: the same colour family, but not a straight swap. Seraphim Flesh is noticeably darker, more saturated, cooler than Guilliman Flesh.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Seraphim Flesh is noticeably darker, more saturated, cooler than Guilliman Flesh.
The same difference applies the other way round: Guilliman Flesh is noticeably lighter, more muted, 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 Seraphim Flesh on the colour wheel
All matches: Guilliman Flesh equivalents · Seraphim Flesh equivalents