Carmine Dragon vs Red Opulence

How close are they?

ΔE 26.4, loose: different enough that you should pick on purpose. Red Opulence is a little darker, warmer than Carmine Dragon.

Can I use Red Opulence instead of Carmine Dragon?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Red Opulence is a little darker, warmer than Carmine Dragon.

Can I use Carmine Dragon instead of Red Opulence?

The same difference applies the other way round: Carmine Dragon is a little 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.