People can have various reasons for such look. It can be a symbol of non-conformity.
Official Rust spaces have a code of conduct that is inclusive and forbids discrimination, and this may attract people who otherwise wouldn’t feel comfortable to participate.
But why purple?
Really hard to tell if this is a question in good faith -- but the answer probably has to do with the fact that purple has been the chosen color of the women's movement since more than a century.
It's a very open minded community and I think that's the reason people can just show who and how they really are.
Attracts*
Based on down votes doesn't seem like community is 'open-minded', I just asked a question I about matter I noticed.
You're the one who is not open minded. You simply could have googled the question. Instead you want to stir up shit, because one some level people who are truly curious aren't going to ask questions in a forum. Only troublemakers do so.