Naggaroth Night vs Phthalo Blue RS

How close are they?

ΔE 9.7, close: similar, with a visible difference side by side. Phthalo Blue RS is more saturated, cooler than Naggaroth Night.

Can I use Phthalo Blue RS instead of Naggaroth Night?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Phthalo Blue RS is more saturated, cooler than Naggaroth Night.

Can I use Naggaroth Night instead of Phthalo Blue RS?

The same difference applies the other way round: Naggaroth Night 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.