Naggaroth Night vs Cerulean Blue Deep

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Cerulean Blue Deep is a little lighter, cooler than Naggaroth Night.

Can I use Cerulean Blue Deep instead of Naggaroth Night?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cerulean Blue Deep is a little lighter, cooler than Naggaroth Night.

Can I use Naggaroth Night instead of Cerulean Blue Deep?

The same difference applies the other way round: Naggaroth Night is a little darker, 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.