ΔE 21.7, loose: different enough that you should pick on purpose. Light Ultramarine Blue is noticeably lighter, much more saturated than Alpha Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Light Ultramarine Blue is noticeably lighter, much more saturated than Alpha Blue.
The same difference applies the other way round: Alpha Blue is noticeably darker, 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.
See Alpha Blue on the colour wheel See Light Ultramarine Blue on the colour wheel
All matches: Alpha Blue equivalents · Light Ultramarine Blue equivalents