ΔE 16.3, loose: different enough that you should pick on purpose. Summersea Blue is a little darker, more saturated, cooler than Alpha Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Summersea Blue is a little darker, more saturated, cooler than Alpha Blue.
The same difference applies the other way round: Alpha Blue is a little lighter, more muted, 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 Alpha Blue on the colour wheel See Summersea Blue on the colour wheel
All matches: Alpha Blue equivalents · Summersea Blue equivalents