ΔE 18.4, loose: different enough that you should pick on purpose. Clear Plum is more saturated, warmer than Naggaroth Night. They are different kinds of paint: Citadel Naggaroth Night is an opaque base/layer paint and Reaper Clear Plum is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Clear Plum is more saturated, warmer than Naggaroth Night. They are different kinds of paint.
The same difference applies the other way round: Naggaroth Night is more muted, 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 Clear Plum on the colour wheel
All matches: Naggaroth Night equivalents · Clear Plum equivalents