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