ΔE 21.1, loose: different enough that you should pick on purpose. Ignis Fatuus is a little lighter, much more saturated, warmer than Fenrisian Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Ignis Fatuus is a little lighter, much more saturated, warmer than Fenrisian Grey.
The same difference applies the other way round: Fenrisian Grey is a little darker, much 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 Fenrisian Grey on the colour wheel See Ignis Fatuus on the colour wheel
All matches: Fenrisian Grey equivalents · Ignis Fatuus equivalents