Scarlet Red vs Plasma Red

How close are they?

ΔE 23.4, loose: different enough that you should pick on purpose. Plasma Red is a little darker, much more muted than Scarlet Red.

Can I use Plasma Red instead of Scarlet Red?

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

Can I use Scarlet Red instead of Plasma Red?

The same difference applies the other way round: Scarlet Red 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.