I know it sounds impossible that I don't know but I never noticed -

On the left side of my steering wheel is the gas gauge. When the tank is full the gauge is all horizontal black lines. As the gas burns the gauge turns white from top to bottom, indicating that the gas level has dropped.

On the right side is the temperature gauge. It is all white. I just got back from the mechanic who said it's supposed to be all white. It's hot here, in the high 80's. He said in the Winter it would have the black bars, not in the Summer when I start warm. He topped all the tanks, checked everything, said I'm good to go.

For whatever reason this does NOT seem to be what I've been looking at for two years, and the manual says nothing about it.

I don't trust MB at all - long story, lawsuit some years ago - and I really don't know the answer.

Does anyone?

Honestly, I know it sounds trite, but if it's working correctly why worry what the colour is..
You could always sell it as a modification from standard ;)

I have visions of working on the road and being stranded.

The manual sounds like the car will explode if something is wrong with the radiator.

MB err of the serious side of cautious as their customers tend to have very DEEP pockets when it comes to litigation, after all the cars cost a small fortune ;)
That and the cost of their "trained specialists" has to be accounted for some how..

That's true. The guy who vaccuumed the car out before they gave it back to me the last time was all in white, complete with gloves.

I thought he was a Physician working a second job!

(I hate this car, by the way.)