A little background... The default display colors are inherited from the method. When you change the colors in the operation, this overrides the inheritance from the method. In the operation, all of the motion type parameters are inherited together - color, feed, and units.
So the way to restore the default color for Rapid is to turn the inheritance back on, by checking the Feed Rapid box in the Details panel in the Operation Navigator, and applying the change with the big green check. But be aware that the rapid feed and units will also be reset to the inherited values.
If you want to use different colors a lot, consider editing the method. For example, if you could change the color of MILL_FINISH to something different, and any operation you create under that method would use that color (and feed).
Attached is a journal that will reset the path display colors in ALL the methods in the work part to the current defaults. Assuming all the operations are inheriting, they will all use the new colors.
Just rename the file from .txt to .vb, and then run it from NX using Menu > Tools > Jounal > Play, or Play on the Developer tab. If you find you need to do this often, I would make a button for it in your UI.