ΔE 23.1, loose: different enough that you should pick on purpose. Sapphire Blue is more muted, cooler than Leviathan Purple. They are different kinds of paint: Citadel Leviathan Purple 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 23). Sapphire Blue is more muted, cooler than Leviathan Purple. They are different kinds of paint.
The same difference applies the other way round: Leviathan Purple is 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 Leviathan Purple on the colour wheel See Sapphire Blue on the colour wheel
All matches: Leviathan Purple equivalents · Sapphire Blue equivalents