Quoted from Jaybird815:And if you look at them from above, the roofs of the trucks aren’t painted, just left white.
if it hasnt already been answered, i think UPS trucks have top white translucent roofs, so to provide natural light into the truck to better identify packages.
man i love the internet...
https://www.grunge.com/172168/the-real-reason-ups-trucks-have-white-roofs/