As far as I understand it, there is not yet support for powershell in the [JavaScript library][1] used by SE to implement this.

See the [meta page][2] about the general feature and [this discussion][3] about Powershell in particular.


  [1]: https://code.google.com/p/google-code-prettify/
  [2]: http://meta.stackexchange.com/questions/184108/what-is-syntax-highlighting-and-how-does-it-work
  [3]: http://meta.stackexchange.com/questions/214232/please-add-support-for-powershell-syntax-highlighting