Like others have said some factories will make color changes for a price or you can get a plain white (no graphics) and take it to a RV paint shop and have it painted.
It's really the type and quality of insulation, windows and air conditioning unit(s) used by the manufacturer that will help keep the inside of the RV cooler. Keep in mind in the middle-east the tents used by nomadic tribes are usually dark colored. It's the cloth the tents are made of and the way they are put together that helps with cooling and heating. Same thing with RVs... design and materials used.