ΔE 13.3, fair: the same colour family, but not a straight swap. Piggy Purple Ink is noticeably lighter than Naggaroth Night. They are different kinds of paint: Citadel Naggaroth Night is an opaque base/layer paint and Formula P3 Piggy 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 13). Piggy Purple Ink is noticeably lighter than Naggaroth Night. They are different kinds of paint.
The same difference applies the other way round: Naggaroth Night is noticeably darker.
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 Piggy Purple Ink on the colour wheel
All matches: Naggaroth Night equivalents · Piggy Purple Ink equivalents