Looking down on another of these trucks today, I was particularly offended that the brown paint ends roughly along the edges. It looks way too much like complacency, not design.

I heard a UPS ad years ago that said the truck roofs were translucent to let in sunlight, so drivers could see packages easily without turning on a light in the truck.

Hey, Glenn!
Yep, the roofs on UPS trucks are white. They're also made out of fiberglass. This combination of color and material is supposed to help keep the interior of the truck cooler. Doesn't work worth a shit, though. There's not enough air movement in the back of the truck in the Summer to allow much cooling to occur (the rear doors and the bulkhead doors are closed for 95% of delivery day - they're only open momentarily when the truck stops and the driver has to enter the back to retrieve packages). Nor can that patch of white on the roof overcome the heat-absorbing mass of brown on the rest of the truck... I've driven those damn things long enough to know! No AC in those trucks either (tractor trailers have AC, though).
Don't know how enlightening this info is, but there ya have it anyway.
Regards,
J.P.