ΔE 13.6, fair: the same colour family, but not a straight swap. Blue Ink is noticeably lighter, more muted, warmer than Soulstone Blue. They are different kinds of paint: Citadel Soulstone Blue is a technical paint or medium and Formula P3 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 14). Blue Ink is noticeably lighter, more muted, warmer than Soulstone Blue. They are different kinds of paint.
The same difference applies the other way round: Soulstone Blue is noticeably darker, 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 Soulstone Blue on the colour wheel See Blue Ink on the colour wheel
All matches: Soulstone Blue equivalents · Blue Ink equivalents