ΔE 13.9, fair: the same colour family, but not a straight swap. Waaagh! Flesh is more saturated, warmer than Ruinous Spell.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Waaagh! Flesh is more saturated, warmer than Ruinous Spell.
The same difference applies the other way round: Ruinous Spell is more muted, cooler.
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 Ruinous Spell on the colour wheel See Waaagh! Flesh on the colour wheel
All matches: Ruinous Spell equivalents · Waaagh! Flesh equivalents