Dark Ultramarine vs Imperial Purple

How close are they?

ΔE 28.9, loose: different enough that you should pick on purpose. Imperial Purple is noticeably lighter, warmer than Dark Ultramarine.

Can I use Imperial Purple instead of Dark Ultramarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Imperial Purple is noticeably lighter, warmer than Dark Ultramarine.

Can I use Dark Ultramarine instead of Imperial Purple?

The same difference applies the other way round: Dark Ultramarine is noticeably darker, cooler.

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.