Rebel Red vs Sanguine Scarlet

How close are they?

ΔE 26.0, loose: different enough that you should pick on purpose. Sanguine Scarlet is more saturated, warmer than Rebel Red.

Can I use Sanguine Scarlet instead of Rebel Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Sanguine Scarlet is more saturated, warmer than Rebel Red.

Can I use Rebel Red instead of Sanguine Scarlet?

The same difference applies the other way round: Rebel Red is more muted, 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.