ΔE 20.5, loose: different enough that you should pick on purpose. Imperial Purple is a little darker, much more saturated, cooler than Spectral Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Imperial Purple is a little darker, much more saturated, cooler than Spectral Purple.
The same difference applies the other way round: Spectral Purple is a little lighter, much 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 Spectral Purple on the colour wheel See Imperial Purple on the colour wheel
All matches: Spectral Purple equivalents · Imperial Purple equivalents