ΔE 13.8, fair: the same colour family, but not a straight swap. Naphthol Red Medium is a little darker, much more saturated than Poppy Red. They are different kinds of paint: Army Painter Poppy Red is a one-coat contrast-style paint and Golden Naphthol Red Medium 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 14). Naphthol Red Medium is a little darker, much more saturated than Poppy Red. They are different kinds of paint.
The same difference applies the other way round: Poppy Red is a little lighter, much more muted.
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 Poppy Red on the colour wheel See Naphthol Red Medium on the colour wheel
All matches: Poppy Red equivalents · Naphthol Red Medium equivalents