I don't know if it's all YC founders or all YC employees, but at least at one point it was public knowledge (as in, Paul Graham repeatedly posted about it) that everyone in YC had an orange username.
[1] https://news.ycombinator.com/item?id=5730720
[2] http://techcrunch.com/2013/05/18/the-evolution-of-hacker-new...
[3] https://hn.algolia.com/?q=If+you+are+a+YC+founder%2C+your+us...
Also, the orange name thing had been discussed on HN long before then. More than once, IIRC. (Sorry, I don't have citations—too tired to dig them up.)