ΔE 12.3, fair: the same colour family, but not a straight swap. Purple Ink is a little darker, warmer than Naggaroth Night. They are different kinds of paint: Citadel Naggaroth Night is an opaque base/layer paint and Reaper Purple Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Purple Ink is a little darker, warmer than Naggaroth Night. They are different kinds of paint.
The same difference applies the other way round: Naggaroth Night is a little lighter, 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 Naggaroth Night on the colour wheel See Purple Ink on the colour wheel
All matches: Naggaroth Night equivalents · Purple Ink equivalents