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