ΔE 16.6, loose: different enough that you should pick on purpose. Primary Cyan is much more saturated than Soulstone Blue. They are different kinds of paint: Citadel Soulstone Blue is a technical paint or medium and Golden Primary Cyan is an opaque base/layer 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). Primary Cyan is much more saturated than Soulstone Blue. They are different kinds of paint.
The same difference applies the other way round: Soulstone Blue is much more muted.
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 Primary Cyan on the colour wheel
All matches: Soulstone Blue equivalents · Primary Cyan equivalents