ΔE 18.8, loose: different enough that you should pick on purpose. Dark Ultramarine is a little darker, much more saturated, warmer than Ultramarine Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Ultramarine is a little darker, much more saturated, warmer than Ultramarine Blue.
The same difference applies the other way round: Ultramarine Blue is a little lighter, much more muted, 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.
See Ultramarine Blue on the colour wheel See Dark Ultramarine on the colour wheel
All matches: Ultramarine Blue equivalents · Dark Ultramarine equivalents