Lucius Lilac vs Ignis Fatuus

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Ignis Fatuus is more saturated, cooler than Lucius Lilac.

Can I use Ignis Fatuus instead of Lucius Lilac?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Ignis Fatuus is more saturated, cooler than Lucius Lilac.

Can I use Lucius Lilac instead of Ignis Fatuus?

The same difference applies the other way round: Lucius Lilac is 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.