A once popular system for covertly indicating one's sexual preferences to prospective partners that involved the use of color-coded handkerchiefs or bandannas worn on one's person or tucked into a pocket. There was no one unified code, but mostly, a handkerchief worn on the left side indicated a dominant/top preference, and when worn on the right, indicated a submissive/bottom preference. Different variations on this system also sometimes assigned different meanings to the various colors, but an example system is given below:

Black - BDSM
Dark Blue - Anal Sex
Light Blue - Oral Sex
Brown - Scat
Green - Prostitution
Grey - Bondage
Orange - Anything Goes
Purple - Piercing
Red - Fisting
Yellow - Watersports

This code is not widely used in modern times.