You can do this in Styler by adding an element called "_pi". That will catch all PIs, and you can color them by modifying the Text Shading attribute.
If you need to be more selective, e.g. use different colors for different PIs, you can do that by adding conditions. Use an XPath condition like this:
self::processing-instruction("NameOfPi")
That will capture all instances of <?NameOfPi?> and apply formatting to them.