Soulstone Blue vs Primer Dark Purple

How close are they?

ΔE 29.6, loose: different enough that you should pick on purpose. Primer Dark Purple is noticeably darker, much more muted, warmer than Soulstone Blue.

Can I use Primer Dark Purple instead of Soulstone Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Primer Dark Purple is noticeably darker, much more muted, warmer than Soulstone Blue.

Can I use Soulstone Blue instead of Primer Dark Purple?

The same difference applies the other way round: Soulstone Blue is noticeably lighter, much 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.