ΔE 18.3, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably lighter, more saturated than Imperial Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ultramarine Blue is noticeably lighter, more saturated than Imperial Blue.
The same difference applies the other way round: Imperial Blue is noticeably darker, more muted.
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 Blue on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Imperial Blue equivalents · Ultramarine Blue equivalents