ΔE 17.3, loose: different enough that you should pick on purpose. Phthalo Blue RS is more saturated, warmer than Imperial Navy.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Phthalo Blue RS is more saturated, warmer than Imperial Navy.
The same difference applies the other way round: Imperial Navy is more muted, cooler.
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 Imperial Navy on the colour wheel See Phthalo Blue RS on the colour wheel
All matches: Imperial Navy equivalents · Phthalo Blue RS equivalents