Red Opulence vs Plasma Red

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Plasma Red is a little darker, much more muted than Red Opulence.

Can I use Plasma Red instead of Red Opulence?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Plasma Red is a little darker, much more muted than Red Opulence.

Can I use Red Opulence instead of Plasma Red?

The same difference applies the other way round: Red Opulence is a little lighter, much 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.