ΔE 26.8, loose: different enough that you should pick on purpose. Dark Ultramarine is noticeably darker, cooler than Genestealer Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Dark Ultramarine is noticeably darker, cooler than Genestealer Purple.
The same difference applies the other way round: Genestealer Purple 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.
See Genestealer Purple on the colour wheel See Dark Ultramarine on the colour wheel
All matches: Genestealer Purple equivalents · Dark Ultramarine equivalents