Well, I was hoping that people could be creative in coming up with uses, but I suppose I can offer a few more ideas.

For instance, maybe in the business world, you might not want to be so overt about being an Effective Altruist because you fear your generosity being taken advantage of, so you might use a subtle variant of these gestures to signal to other Effective Altruists who you are, without giving it away to more egoistic types.

Alternatively, it could be used to display your affiliation in such a way that signals to people in, say, an audience during a speech or lecture, where you're coming from. Again, this can be overt, or covert depending on circumstances.

Furthermore, if this is a one on one "conversation", the response could be useful for telling you how overt or covert you should be in the environment. Say for instance, you display a subtle "Dark" gesture to someone you suspect to be an Effective Altruist in an environment that may otherwise be hostile to Effective Altruism (like say, a financial company). Depending on their response, you can gauge how open you should be in the future. They might for instance, give you a very covert sign in return, which may signal that the environment is hostile. Alternatively, they may signal back with the "Light" gesture, indicating that they themselves are able to be open in this environment safely.

While it is true that most of us want to be open as effective altruists, I suspect that there is a significant number of people who while sympathetic to our causes, are hesitant to openly affiliate for fear of being taken advantage of by free riders and egoists. These gestures would be most useful for those people.