Necromancer Purple vs Regal Blue

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Regal Blue is more saturated, cooler than Necromancer Purple.

Can I use Regal Blue instead of Necromancer Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Regal Blue is more saturated, cooler than Necromancer Purple.

Can I use Necromancer Purple instead of Regal Blue?

The same difference applies the other way round: Necromancer Purple is more muted, 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.