The sketch elements would not show as fully defined (black).
Knowing that there used to be a button called "Relationship Colors" I searched the "view" tab of the ribbon in vain. Then I typed "Colors" into the command finder, which then told me that there were no buttons which include that word in the name in the current environment. I click "search outside of environment" which confirms that indeed, there is a button called that, but it is only found in the environment: "( and others)"
I finally found it myself, right there on the ribbon. Thanks, Command Finder.
WHY IS THE BUTTON GROUPED WITH INTELLISKETCH OPTIONS?!??!?!
Also, this help file sentence is word soup to me:
"When you set the Sketch Relationship Colors option, the Line Color button on all create or edit 2D element and Select Tool command bars is not available and you cannot apply or change element colors."