ΔE 23.3, loose: different enough that you should pick on purpose. Light Ultramarine Blue is a little lighter, more saturated, warmer than Ionic Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Light Ultramarine Blue is a little lighter, more saturated, warmer than Ionic Blue.
The same difference applies the other way round: Ionic Blue is a little darker, 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 Ionic Blue on the colour wheel See Light Ultramarine Blue on the colour wheel
All matches: Ionic Blue equivalents · Light Ultramarine Blue equivalents