Calgar Blue vs Cerulean Blue Deep

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Cerulean Blue Deep is noticeably darker, more muted than Calgar Blue.

Can I use Cerulean Blue Deep instead of Calgar Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Cerulean Blue Deep is noticeably darker, more muted than Calgar Blue.

Can I use Calgar Blue instead of Cerulean Blue Deep?

The same difference applies the other way round: Calgar Blue is noticeably lighter, more saturated.

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.