ΔE 26.4, loose: different enough that you should pick on purpose. Light Ultramarine Blue is more saturated, warmer than Blue Flux.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Light Ultramarine Blue is more saturated, warmer than Blue Flux.
The same difference applies the other way round: Blue Flux is 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 Blue Flux on the colour wheel See Light Ultramarine Blue on the colour wheel
All matches: Blue Flux equivalents · Light Ultramarine Blue equivalents