We've got a black trailer now and when I buy a new trailer it WILL NOT be black (even though I 100% prefer that color). The only reason is the heat. I'm running a 13.5 AC in a 7 x 16 enclosed that is mostly insulated, and unless you get to the site early in the day, the trailer is a sauna until after dark.
Once a black trailer is hot, the amount of thermal energy I'm trying to dump out of it takes forever with the A/C. Yes, it will start to cool it, but its built up too much on a 80+ degree day. Overnight, it'll have that thing like an icebox (into the 50's). The next day, as soon as the sun comes up, the temp starts to creep up until we're pushing 70's by mid-day.
Go with a light color if at all possible (or you're running two AC units)
Border War Smokers - "BBQ worth killin' your neighbor for"