ΔE 16.3, loose: different enough that you should pick on purpose. Elfic Blue is a little darker, cooler than Ignis Fatuus.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Elfic Blue is a little darker, cooler than Ignis Fatuus.
The same difference applies the other way round: Ignis Fatuus 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 Ignis Fatuus on the colour wheel See Elfic Blue on the colour wheel
All matches: Ignis Fatuus equivalents · Elfic Blue equivalents