It's color-anodized aluminum, a process more like dying than painting. Combines anti-corrosionizing with coloring. Same thing used on those purty aluminum drinking glasses from the fifties. Since the process is so specialized, I doubt you could duplicate the look with a do-it-yourself product.
It looks like the color loss was due to some chemical washing across it....if it's uniformity you're looking for I s'pose you could strip it off the rest of the trailer, too. That would likely lead to more oxidizing of the type you're already seeing where the color's gone, though.
