Flat Red vs Scarlet Blood

How close are they?

ΔE 7.1, close: similar, with a visible difference side by side. Scarlet Blood is a little darker, more muted than Flat Red.

Can I use Scarlet Blood instead of Flat Red?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Scarlet Blood is a little darker, more muted than Flat Red.

Can I use Flat Red instead of Scarlet Blood?

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