Ignis Fatuus vs Alien Purple

How close are they?

ΔE 24.9, loose: different enough that you should pick on purpose. Alien Purple is noticeably darker, much more saturated than Ignis Fatuus.

Can I use Alien Purple instead of Ignis Fatuus?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Alien Purple is noticeably darker, much more saturated than Ignis Fatuus.

Can I use Ignis Fatuus instead of Alien Purple?

The same difference applies the other way round: Ignis Fatuus is noticeably lighter, much more muted.

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.