You don't need a macro for this. The colour of the placeholder text of a content control is dependent on the built-in placeholder text character style. You can modify the colour of that style from grey to red in that document.
If you want to do that with a macro, you need the line
Code:
ActiveDocument.Styles("Placeholder Text").Font.Color = wdColorRed
You may find
https://www.gmayor.com/insert_content_control_addin.htm useful