ΔE 11.7, fair: the same colour family, but not a straight swap. Blue Ink is a little darker, more muted, cooler than Night Lords Blue. They are different kinds of paint: Citadel Night Lords Blue is an opaque base/layer paint and Reaper Blue Ink 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). Blue Ink is a little darker, more muted, cooler than Night Lords Blue. They are different kinds of paint.
The same difference applies the other way round: Night Lords Blue is a little lighter, more saturated, 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 Night Lords Blue on the colour wheel See Blue Ink on the colour wheel
All matches: Night Lords Blue equivalents · Blue Ink equivalents