I found these:

~Red Bandana (gang talk)

"When a thug ass Blood is gangbanging, he often sports a red bandana to alert fellow American citizens of his "Thug Nature" and his affiliation to bloods, esp. LA Pirus. If you wear blue around these folks, they will blow your head off."

~"What does a red bandana around your neck mean?" (miner talk)

"During this guerrilla war in the Appalachian coalfields, 10,000+ striking miners wore red bandanas around their necks to identify themselves on the battlefield against the white-armband-wearing, industry-funded opposition."

~Or this: Handkerchief code (queer code)

https://en.wikipedia.org/wiki/Handkerchief_code

"Red bandana = 'Fisting'"