Just a bit of follow-up trivia and credit where it’s due… 
On the xonsh settings URL you referred me to, the check-mark indicating that the Syntax Highlighting feature is supported via the prompt-toolkit Python library; well that prompt-toolkit library was years-ago written by Jonathan Slenders (very nice guy), and that’s what actually drives the rich interactivity of ptpython(1), also written by Jonathan.
It all ties together. 