ΔE 11.8, fair: the same colour family, but not a straight swap. Phthalo Blue (Red Shade) is a little darker, more muted, warmer than Drakenhof Nightshade. They are different kinds of paint: Citadel Drakenhof Nightshade is a wash/shade and Golden Phthalo Blue (Red Shade) 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). Phthalo Blue (Red Shade) is a little darker, more muted, warmer than Drakenhof Nightshade. They are different kinds of paint.
The same difference applies the other way round: Drakenhof Nightshade is a little lighter, more saturated, 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 Drakenhof Nightshade on the colour wheel See Phthalo Blue (Red Shade) on the colour wheel
All matches: Drakenhof Nightshade equivalents · Phthalo Blue (Red Shade) equivalents