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.
One more behavior: I took a survey (which I can't find again) about hugging from behind, and everyone who answered hated it, except for a few who had a short list of people who they permitted it from.
I didn't have a random or especially large sample, but the unanimity was striking.
One of the lessons highlighted in the thread "Less Wrong NYC: Case Study of a Successful Rationalist Chapter" is Gender ratio matters.
There have recently been a number of articles addressing one social skills issue that might be affecting this, from the perspective of a geeky/sciencefiction community with similar attributes to LessWrong, and I want to link to these, not just so the people potentially causing problems get to read them, but also so everyone else knows the resource is there and has a name for the problem, which may facilitate wider discussion and make it easier for others to know when to point towards the resources those who would benefit by them.
However before I do, in the light of RedRobot's comment in the "Of Gender and Rationality" thread, I'd like to echo a sentiment from one of the articles, that people exhibiting this behaviour may be of any gender and may victimise upon any gender. And so, while it may be correlated with a particular gender, it is the behaviour that should be focused upon, and turning this thread into bashing of one gender (or defensiveness against perceived bashing) would be unhelpful.
Ok, disclaimers out of the way, here are the links:
Some of those raise deeper issues about rape culture and audience as enabler, but the TLDR summary is:
EDITED TO ADD:
Despite the way some of the links are framed as being addressed to creepers, this post is aimed at least as much at the community as a whole, intended to trigger a discussion on how the community should best go about handling such a problem once identified, with the TLDR being "set of restraints to place on someone who is burning the commons", rather that a complete description that guarantees that anyone who doesn't meet it isn't creepy. (Thank you to jsteinhardt for clearly verbalising the misinterpretation - for discussion see his reply to this post)