Cerulean Blue Deep vs Royal Purple

How close are they?

ΔE 25.0, loose: different enough that you should pick on purpose. Royal Purple is a little darker, more muted, warmer than Cerulean Blue Deep.

Can I use Royal Purple instead of Cerulean Blue Deep?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Royal Purple is a little darker, more muted, warmer than Cerulean Blue Deep.

Can I use Cerulean Blue Deep instead of Royal Purple?

The same difference applies the other way round: Cerulean Blue Deep is a little lighter, more saturated, 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.