ΔE 6.3, close: similar, with a visible difference side by side. Infantry Blue is a little darker, cooler than Nightshade Purple.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Infantry Blue is a little darker, cooler than Nightshade Purple.
The same difference applies the other way round: Nightshade Purple is a little lighter, 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 Nightshade Purple on the colour wheel See Infantry Blue on the colour wheel
All matches: Nightshade Purple equivalents · Infantry Blue equivalents