BDSM practitioners or casual sex seekers sometimes indicate whether they are a top or bottom, and what kind of sex they are looking for, by color-coded handkerchiefs, worn on the left side of the body for tops and the right for bottoms.
There is no universally understood color code. People generally agree upon the more common practices, but there is no consensus on the more uncommon techniques.
Hanky codes are most used by gay men, and the practice is now largely out of fashion.
From On the Safe Edge: A Manual for SM Play, by Trevor Jacques (ISBN 1-895857-05-8)
The hanky code is a color code for indicating sexual preferences among homosexuals, sado-masochists and others. The color of a handkerchief or bandana worn in the back trouser pocket indicates the type of sexual behavior sought, and the choice of pocket indicates whether the person seeks to be the active or passive partner in that behavior.
At this point, hanky codes are a memory, although they were once quite popular.
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