Usually, the color green means that one is single and looking for a relationship. Red means that one is currently in a relationship, or not looking for one. Yellow may mean "unsure" or "maybe," or could mean that one is in a relationship but still open to advances.
The purpose of a stop light party is to decrease the apprehension associated with approaching potential mates at parties. According to one Albany-area DJ, it "takes the guesswork out of the night." It also serves the purpose of providing an easy indicator of one's unavailability to fend off unwelcome advances.
Guests at a stop light party might wear different-colored glow-in-the-dark jewelry, hold glow sticks, or incorporate stop light colors into their outfits.