Sin, and the blood of the Sacrifice that cleanses for Sin.
Info and Details:
Red as Blood – 2 Kings 3:22
Sins and Scarlet Isaiah 1:18
Esau (Edom) is Red – Genesis 25:25, 30 See Strongs # H123
123. אֱדֹם ʾĔdôm, ed-ome´; or (fully) אֱדוֹם ʾÊdôwm, ed-ome´; from 122; red [see Genesis 25:25]; Edom, the elder twin-brother of Jacob; hence the region (Idumæa) occupied by him:—Edom, Edomites, Idumea.