We would want to show a different color on the selected part other than the default dodgerblue color. We would like to do this for any and all selections in the application. Can you please let us know how we can achieve this?
First, please read GoJS Selection -- Northwoods Software.
You probably want to set Diagram | GoJS API. The default implementation is given in the setupSelectionAdornments function in https://gojs.net/latest/extensions/templates.js.