Q:

What is the origin of the phrase "caught red-handed"?

I'm just wondering: why "red"?

A:

On etymonline you will find that it is, presumably, from the blood on hands.
There are other more detailed articles around, quote:

Red-handed doesn't have a mythical origin however - it is a straightforward allusion to having blood on one's hands after the execution of a murder or a poaching session. The term originates, not from Northern Ireland, but from a country not so far from there, socially and geographically, i.e. Scotland. An earlier form of 'red-handed', simply 'red hand', dates back to a usage in the Scottish Acts of Parliament of James I, 1432.
Red-hand appears in print many times in Scottish legal proceedings from the 15th century onward. For example, this piece from Sir George Mackenzie's A discourse upon the laws and customs of Scotland in matters criminal, 1674:
"If he be not taken red-hand the sheriff cannot proceed against him."

