ΔE 11.7, fair: the same colour family, but not a straight swap. Transparent Magenta is a little darker, cooler than Astorath Red. They are different kinds of paint: Citadel Astorath Red is an opaque base/layer paint and Green Stuff World Transparent Magenta 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). Transparent Magenta is a little darker, cooler than Astorath Red. They are different kinds of paint.
The same difference applies the other way round: Astorath Red is a little lighter, warmer.
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 Astorath Red on the colour wheel See Transparent Magenta on the colour wheel
All matches: Astorath Red equivalents · Transparent Magenta equivalents