Ignis Fatuus vs Ultramarine Blue

How close are they?

ΔE 25.0, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably darker, more saturated, cooler than Ignis Fatuus.

Can I use Ultramarine Blue instead of Ignis Fatuus?

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

Can I use Ignis Fatuus instead of Ultramarine Blue?

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