ΔE 18.8, loose: different enough that you should pick on purpose. Slate Grey is a little darker, much more muted than Warpfiend Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Slate Grey is a little darker, much more muted than Warpfiend Grey.
The same difference applies the other way round: Warpfiend Grey is a little lighter, much more saturated.
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 Slate Grey on the colour wheel
All matches: Warpfiend Grey equivalents · Slate Grey equivalents