Necromancer Purple vs Flat Blue

How close are they?

ΔE 9.5, close: similar, with a visible difference side by side. Flat Blue is cooler than Necromancer Purple.

Can I use Flat Blue instead of Necromancer Purple?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Blue is cooler than Necromancer Purple.

Can I use Necromancer Purple instead of Flat Blue?

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