ΔE 27.9, loose: different enough that you should pick on purpose. Ultramarine Violet is noticeably lighter, cooler than Kakophoni Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Ultramarine Violet is noticeably lighter, cooler than Kakophoni Purple.
The same difference applies the other way round: Kakophoni Purple is noticeably darker, 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 Kakophoni Purple on the colour wheel See Ultramarine Violet on the colour wheel
All matches: Kakophoni Purple equivalents · Ultramarine Violet equivalents