ΔE 23.5, loose: different enough that you should pick on purpose. Ultramarine Blue is a little lighter, much more muted, cooler than Kantor Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Ultramarine Blue is a little lighter, much more muted, cooler than Kantor Blue.
The same difference applies the other way round: Kantor Blue is a little 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 Kantor Blue on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Kantor Blue equivalents · Ultramarine Blue equivalents