I want to try lightening the color of the van's roof to see if it helps keep the interior cool.  

I thought the white vinyl (?) sheets I've seen affixed to horizontal surfaces of new cars might work.  As with the new cars, this must be a temporary installation.  Anyone know of any sources or alternatives?

Paint is a medium brown metallic. I've already added window tint.  The a/c doesn't get cold the vehicle cold enough or me in the Desert Summer.  

By the way:  I recently saw a permanent solution offered by Canepa Designs at Canepa.com  They advertise a 15% reduction (presumably from Black)

Thanks.

Reflective UV "foil" material.. on the inside of the ceiling.  I removed my liner and had it installed.. I've noticed a big difference

Reflective UV foil inside a the ceiling sounds like a automotive tin-foil-hat!  I'll look into that too.
Thanks.

For the white vinyl, try [url=http://www.window-cling.com/Blank_static_vinyl.htm]here[/url]. You can also go to a shop that makes vinyl decals for cars and trucks, and ask them where to get a sheet of the static stuff. They may even have it. I suggest applying the vinyl in sections to your van roof, as working with a big sheet will by a major pain.

I like the tinfoil hat for the car idea, though. Link anyone?

Edited to fix my sloppy board code. That's about the fifth time today I've muffed the link code.

Years ago we did a similar job on a friends van, we used styrofoam sheets and after removing the headliner put contact cement on the roof and sheets ( I think they were 3/8 and slightly flexible ) and just stuck them up, it made a big difference.
Good luck. 'Guzzi out.

Duct Tape

How about cutting the top off altogether? Nice breeze.