ΔE 10.2, fair: the same colour family, but not a straight swap. Blue Ink is a little lighter, cooler than Dark Blue Tone. They are different kinds of paint: Army Painter Dark Blue Tone is a wash/shade 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 10). Blue Ink is a little lighter, cooler than Dark Blue Tone. They are different kinds of paint.
The same difference applies the other way round: Dark Blue Tone is a little darker, 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 Dark Blue Tone on the colour wheel See Blue Ink on the colour wheel
All matches: Dark Blue Tone equivalents · Blue Ink equivalents