ΔE 19.2, loose: different enough that you should pick on purpose. Faded Ultramarine is a little darker, much more saturated, warmer than Fenrisian Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Faded Ultramarine is a little darker, much more saturated, warmer than Fenrisian Grey.
The same difference applies the other way round: Fenrisian Grey is a little lighter, 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 Faded Ultramarine on the colour wheel
All matches: Fenrisian Grey equivalents · Faded Ultramarine equivalents