Most street legal vehicles utilize a combination of three colored lights: white for headlights and reverse lights, red for tail and brake lights, and orange for blinkers....
Mercedes-Benz debuts turquoise exterior lights to indicate the car is self-driving | A visual indicator for other drivers::undefined
Same reason at this point as "Student Driver" bumper stickers - so you know it's inexperienced and may behave weirdly, so maybe keep a bit more distance than usual or something.
I would love to have an indicator for adaptive cruise control because the way it only reacts to the car right in front of you rather aggressively means it causes shockwave traffic jams unless the human driver behind you keeps enough distance.
The ACC in my car maintains a good bit of distance to the car ahead and doesn't respond suddenly to things coming closer than that distance. I've rarely if ever seen it brake inappropriately.