Alaitoc Blue vs Cerulean Blue Deep

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Cerulean Blue Deep is noticeably darker, warmer than Alaitoc Blue.

Can I use Cerulean Blue Deep instead of Alaitoc Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Cerulean Blue Deep is noticeably darker, warmer than Alaitoc Blue.

Can I use Alaitoc Blue instead of Cerulean Blue Deep?

The same difference applies the other way round: Alaitoc Blue is noticeably lighter, cooler.

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.