ΔE 12.2, fair: the same colour family, but not a straight swap. Fluor Orange is a little lighter, warmer than Poppy Red. They are different kinds of paint: Army Painter Poppy Red is a one-coat contrast-style paint and Green Stuff World Fluor Orange 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). Fluor Orange is a little lighter, warmer than Poppy Red. They are different kinds of paint.
The same difference applies the other way round: Poppy Red is a little darker, 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 Poppy Red on the colour wheel See Fluor Orange on the colour wheel
All matches: Poppy Red equivalents · Fluor Orange equivalents