Space Balls: Ultima Thule by the colours

 Ultima Thule flyby gives up some colour.

The Dark patch is Nitrogen Dioxide, or Cobolt Ammine.

Updated (6/1/2019): The light patch between both objects is Nitrogen dioxide.
The dark patch on the upper spheroid is 'Cobolt Ammine'.

That Nitrogen is a common factor in this colour band gives us a Nitrogen rich surface.

 So best bet:

• Nitrogen Dioxide
• Nitrogen Trioxide
• Nitrogen Trihydride

And a resting toucher...