Wild guess: does that depend on driver profile, and would not work under guest profile ? Can you use the search function to search for colour settings of the ambient light? Yours, Pixie
For me, the setting wasn't visible in the menu at first either. I had to disconnect the car battery for the menu item to appear. Five minutes of disconnection should be enough.
I read about this solution on the internet and couldn't believe it worked.
A reset of the Multimedia Unit via the small hole near the left turning knob would probably have resulted in the same. 
Pressing that little Button just results in a warm start of the AVU. IIRC this will not bring back the mentioned setting once it has disappeared - this requires a cold start, thus disconnection of 12V-Battery.
A reset of the Multimedia Unit via the small hole near the left turning knob would probably have resulted in the same.
I tried that first and had no success.
