ΔE 17.9, loose: different enough that you should pick on purpose. Ignis Fatuus is more saturated, warmer than Etherium Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ignis Fatuus is more saturated, warmer than Etherium Blue.
The same difference applies the other way round: Etherium Blue is 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 Etherium Blue on the colour wheel See Ignis Fatuus on the colour wheel
All matches: Etherium Blue equivalents · Ignis Fatuus equivalents