ΔE 10.2, fair: the same colour family, but not a straight swap. Demon Red is a little lighter than Spiritstone Red. They are different kinds of paint: Citadel Spiritstone Red is a technical paint or medium and Nocturna Models Demon Red is an opaque base/layer 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 10). Demon Red is a little lighter than Spiritstone Red. They are different kinds of paint.
The same difference applies the other way round: Spiritstone Red is a little 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 Spiritstone Red on the colour wheel See Demon Red on the colour wheel
All matches: Spiritstone Red equivalents · Demon Red equivalents