I imagine that one would post some kind of notice at the entrance to the venue, which would precisely explain the mapping of bracelet colors to the norms of personal interaction that they indicate; the bracelets themselves would then be piled up in baskets beneath that notice.
I think it’s important to have explicit norms of permissible behavior for a few reasons:
The juridical function of explicit norms: Explicit norms form a bright line by which to identify malicious actors, such as, for example, an aggressive pickup artist who is willing to harass dozens of women in order to somewhat increase the likelihood that he will take one home with him.
The didactic function of explicit norms: Explicit norms allow well-intentioned people to clearly distinguish behaviors that are acceptable to the community from those that are harmful and forbidden. For example, in the absence of explicit norms, a man who lacked social skill might not realize that putting his hands on a woman’s shoulders would probably make her uneasy. At the same time, however, he might also be refraining from certain pleasurable actions because he wrongly feared them to be unacceptable to the community, when, in fact, they were really innocuous.
The efficiency function of explicit norms: Explicit norms make it easier for people to determine which norms to apply in their relations with one another. In the absence of explicit norms, people must invest significant amounts of time trying to identify other people with whom they have complementary interests. A bracelet-signalling scheme would lessen the burden of these taxing social negotiations.
In practice, I think a relatively open gathering at a public location would call for the following levels of intimacy:
Black bracelet: I’m not interested in anything except polite intellectual discussion.
White bracelet: You may try to establish a more personal relationship with me, but if I’m not interested in you, I expect you to take notice of this quickly and to gracefully withdraw.
Purple bracelet: Feel free to approach me and to say or propose anything you like. It won’t bother me very much, and if I don’t enjoy your company, I’ll frankly tell you to go away.
On the other hand, a debauched soirée at a private house might require a rather different scheme, such as:
Black bracelet: I’m not interested in anything except polite intellectual discussion.
White bracelet: You may say what you like to me, but no touching. If I tell you to back off, do so immediately.
Purple bracelet: You may hug me, put your arms around my shoulders, and so on, but not in any way that is grossly erotic. If I tell you to stop, do so immediately.
Green bracelet: You may wrestle with me, grope me, kiss my body, etc. If I tell you to stop, do so immediately.
It might be a good idea to require each person to choose and wear a colored bracelet, such that the color of the bracelet would signal to other parties the level of decorum that they must apply in their relations with its wearer. For example:
A black blacelet indicates that all relations with its wearer must remain scrupulously reserved.
A white bracelet indicates that prior consent must politely be obtained from its wearer before intimate or informal relations are initiated; if this consent is refused, or later revoked, the other party must apologize and desist immediately.
A purple bracelet indicates that its wearer may at all times be presumed to consent to the maximum level of intimacy or informality that is permitted at the gathering.
Obviously, many permutations of and enhancements to this scheme are conceivable.