ΔE 31.1, loose: different enough that you should pick on purpose. Sapphire Blue is much more muted, cooler than Purple Swarm. They are different kinds of paint: Army Painter Purple Swarm is a one-coat contrast-style paint and Reaper Sapphire Blue 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 31). Sapphire Blue is much more muted, cooler than Purple Swarm. They are different kinds of paint.
The same difference applies the other way round: Purple Swarm is much more saturated, 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 Purple Swarm on the colour wheel See Sapphire Blue on the colour wheel
All matches: Purple Swarm equivalents · Sapphire Blue equivalents