One of the things historians pick at in Tombstone is the Cowboy Gang wearing red sashes as an identifier, ala Crips and Bloods. This is not true of the Arizona Cowboy Gang, but in the 1890s the Wyoming Stock Growers Association claimed there was a Red Sash Gang rustling cattle...

...Nate Champion, the character Christopher Walken plays in Heaven's Gate, was said to be the head of this gang, although historians doubt the Association's claims... Attached is a photo of Nate Champion (he is the mustachioed man with the light colored bandana)...

I've been reading Banditti of the Plains, a contemporary account of the Johnson County War, written by a man who worked for the Association and then switched sides after their murderous actions. The Association had it blocked from publication for years...

...in it he describes Nate Champion's dead body as wearing a red sash. I don't know if this was a gang identifier or if it was simply fashion. There are contemporary accounts of (non-rustling) cowboys wearing sashes but few photos...

So of course I tried to research what influenced this fashion choice among cowboys. Western artist Charlie Russell often depicted cowboys wearing them and since he lived during the height of the cowboy era his work is regarded as highly authentic...

Some have claimed that it became popular after Wild Bill Hickok, who was said to wear two Navy Colts butt first in his red sash. This is the reason Kevin Jarre gives in his Tombstone script for the Cowboy's fashion. But of the many photos of Hickok, none show him with a sash...

And the sash that Charlie Russell wore, on closer inspection, had a design that came from the Métis people of Canada, a multi-ancestral group descended from indigenous North Americans and European settlers. There was also a Métis tribe in Montana. Did Russell adopt it from them?

As for Hickok, if he did indeed wear a red sash, my guess is that it would have been based on Civil War military fashion, especially considering that the belt he is pictured with is a military belt...

As for cowboys wearing sashes in the Southwest, my thinking is that they adopted it from Mexican vaqueros (buckaroos), like so many other things. Here's a photo of some Californios and a few contemporary paintings of pre-Civil War vaqueros:

Canadian or Northwestern U.S. cowboys, on the other hand, probably got their sashes from the aforementioned Métis people. Here are some 19th century photos of Canadians wearing sashes:

This has been Old West Clothing Pedantry for the day. Allow William Surrey Hart to play us out.