ΔE 13.2, fair: the same colour family, but not a straight swap. Grey Blue is a little darker, cooler than Warpfiend Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Grey Blue is a little darker, cooler than Warpfiend Grey.
The same difference applies the other way round: Warpfiend Grey is a little lighter, 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 Warpfiend Grey on the colour wheel See Grey Blue on the colour wheel
All matches: Warpfiend Grey equivalents · Grey Blue equivalents