ΔE 20.7, loose: different enough that you should pick on purpose. Imperial Blue is a little lighter, much more muted, cooler than Dark Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Imperial Blue is a little lighter, much more muted, cooler than Dark Blue.
The same difference applies the other way round: Dark Blue is a little darker, 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 Dark Blue on the colour wheel See Imperial Blue on the colour wheel
All matches: Dark Blue equivalents · Imperial Blue equivalents