ΔE 9.5, close: similar, with a visible difference side by side. Violet Grey is a little lighter, cooler than Warpfiend Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Violet Grey is a little lighter, cooler than Warpfiend Grey.
The same difference applies the other way round: Warpfiend Grey is a little darker, 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 Violet Grey on the colour wheel
All matches: Warpfiend Grey equivalents · Violet Grey equivalents