ΔE 35.4, loose: different enough that you should pick on purpose. Ignis Fatuus is a little lighter, more muted, cooler than Emperor's Children.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). Ignis Fatuus is a little lighter, more muted, cooler than Emperor's Children.
The same difference applies the other way round: Emperor's Children is a little darker, more saturated, 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 Emperor's Children on the colour wheel See Ignis Fatuus on the colour wheel
All matches: Emperor's Children equivalents · Ignis Fatuus equivalents