It can sometimes be partially red. For instance, in some modes or conditions the side sensors will be inoperable, so the icon will be all green except the two bars to each side, which will be red.
And yes, when the light gets low enough, you will get that message and the sensors will be off or inoperable. This is dicey flying if you're accustom to always having the sensors warn you of obstacles, plus your own eyes will see less as well. Your visual feed may be your best safety in such cases: before moving backwards / sideways, turn to point the AC in that direction so you can see if there are obstacles with the camera.
Chris