ΔE 24.5, loose: different enough that you should pick on purpose. Light Ultramarine is much more saturated than Ultramarine Blue.
Can I use Light Ultramarine instead of Ultramarine Blue?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Light Ultramarine is much more saturated than Ultramarine Blue.
Can I use Ultramarine Blue instead of Light Ultramarine?
The same difference applies the other way round: Ultramarine Blue is much more muted.
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.