Blood Red Skies vs Scarlet

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Scarlet is noticeably darker, much more muted than Blood Red Skies.

Can I use Scarlet instead of Blood Red Skies?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Scarlet is noticeably darker, much more muted than Blood Red Skies.

Can I use Blood Red Skies instead of Scarlet?

The same difference applies the other way round: Blood Red Skies is noticeably 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.