ΔE 17.7, loose: different enough that you should pick on purpose. Ultramarine is a little darker, much more saturated, cooler than Cultist Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ultramarine is a little darker, much more saturated, cooler than Cultist Purple.
The same difference applies the other way round: Cultist Purple is a little lighter, much more muted, 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 Cultist Purple on the colour wheel See Ultramarine on the colour wheel
All matches: Cultist Purple equivalents · Ultramarine equivalents