I assume you are using a fully shaded view?
In the view settings -> shading, click on the "shaded cut face color" this will bring up the color dialog box; press "control" and click on the currently selected color. This will deselect the color and cause NX to inherit the cut face color from the body.
Thnx a lot. Can it be that you have to use the deselect all color button when in the shaded cut face color dialog box. This works for me. The ctrl key didn't work for me?