ΔE 10.0, fair: the same colour family, but not a straight swap. Ink Dark Turquoise is a little darker, warmer than Dark Reaper. They are different kinds of paint: Citadel Dark Reaper is an opaque base/layer paint and Vallejo Ink Dark Turquoise 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 10). Ink Dark Turquoise is a little darker, warmer than Dark Reaper. They are different kinds of paint.
The same difference applies the other way round: Dark Reaper is a little lighter, 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 Dark Reaper on the colour wheel See Ink Dark Turquoise on the colour wheel
All matches: Dark Reaper equivalents · Ink Dark Turquoise equivalents