ΔE 17.4, loose: different enough that you should pick on purpose. Purple Ink is more saturated, warmer than Dark Blue Tone. They are different kinds of paint: Army Painter Dark Blue Tone is a wash/shade and Reaper Purple Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Purple Ink is more saturated, warmer than Dark Blue Tone. They are different kinds of paint.
The same difference applies the other way round: Dark Blue Tone is more muted, 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 Blue Tone on the colour wheel See Purple Ink on the colour wheel
All matches: Dark Blue Tone equivalents · Purple Ink equivalents