ΔE 25.4, loose: different enough that you should pick on purpose. Imperial Blue is noticeably lighter, much more muted, cooler than Dark Ultramarine.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Imperial Blue is noticeably lighter, much more muted, cooler than Dark Ultramarine.
The same difference applies the other way round: Dark Ultramarine is noticeably darker, much more saturated, 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.
See Dark Ultramarine on the colour wheel See Imperial Blue on the colour wheel
All matches: Dark Ultramarine equivalents · Imperial Blue equivalents