In a recent search of unknown settings I discovered that in the App settings of a project on the tab 'Miscellaneous' you can set Activity Default Colors. I happily changed that every external activity should have a purple default color so I recognize these activities immediately in my microflow.
At Bizzomate we have the guideline that when you use an activity which handles a non-persistent object you give it the background-color 'Orange' (or Yellow would be better though). Wouldn't this be a nice setting to add to these Activity Default Colors?
So whenever you use an activity that handles a non-persistent object it automatically colors yellow/orange or whatever color you have chosen in these Activity Default Color settings.
The same would be usable for activities related to External objects.
I hope I made my idea clear :-)