Eidolon Purple Clear vs Dark Ultramarine

How close are they?

ΔE 26.8, loose: different enough that you should pick on purpose. Dark Ultramarine is noticeably darker, cooler than Eidolon Purple Clear.

Can I use Dark Ultramarine instead of Eidolon Purple Clear?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Dark Ultramarine is noticeably darker, cooler than Eidolon Purple Clear.

Can I use Eidolon Purple Clear instead of Dark Ultramarine?

The same difference applies the other way round: Eidolon Purple Clear is noticeably lighter, 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.