I've done that in both my vans. I like how it looks, specially on the Chateau as it matches the rockers. on my 2000 Chateau I had the same problem than you; needed to fix some rust on the rain gutters.
I painted using Rustolleum and a roller. Far from view, it looked good.
The 2010 was silver before I painted brown and black.
Living in Houston, TX heat management is a priority. I haven't done any scientific measurement, but I want to believe that it works.