Sanguine Scarlet vs Flat Red

How close are they?

ΔE 8.6, close: similar, with a visible difference side by side. Flat Red is a little lighter, more muted than Sanguine Scarlet.

Can I use Flat Red instead of Sanguine Scarlet?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Red is a little lighter, more muted than Sanguine Scarlet.

Can I use Sanguine Scarlet instead of Flat Red?

The same difference applies the other way round: Sanguine Scarlet is a little darker, 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.