ΔE 33.6, loose: different enough that you should pick on purpose. Ignis Fatuus is noticeably lighter, much more muted, warmer than Crystal Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Ignis Fatuus is noticeably lighter, much more muted, warmer than Crystal Blue.
The same difference applies the other way round: Crystal Blue is noticeably darker, much more saturated, 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 Crystal Blue on the colour wheel See Ignis Fatuus on the colour wheel
All matches: Crystal Blue equivalents · Ignis Fatuus equivalents