ΔE 11.3, fair: the same colour family, but not a straight swap. Slaanesh Grey is a little darker, warmer than French Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Slaanesh Grey is a little darker, warmer than French Blue.
The same difference applies the other way round: French Blue is a little lighter, 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 French Blue on the colour wheel See Slaanesh Grey on the colour wheel
All matches: French Blue equivalents · Slaanesh Grey equivalents